Is there a need for the existence of this specific Trunks' timeline?

[ahill1]

@dbgtFO --> only a doubt and I'd like to hear your opinion. How do you think Trunks [from Cell's timeline] killed the androids if he didn't travel to the past? He couldn't have possibly found out about the underground laboratory and get the blue prints since he wouldn't know about Cell.

More to the point, the anime shows us directly the Trunks encounters Cell after defeating the Androids (and not via some blueprints, rather through pure strength ass-kickery), plus a time gap where the world begins to recover.

What? How does the anime show Trunks killing the androids without a shutdown controller? I am not doubting you, just interested in seeing where that's implied as I haven't seen the anime in a long time.

Also, I'd like to see @dbgtFO's opinion on this. He, like me, doesn't see the need for the existence of this unseen timeline, so I'd like to see his opinion on how the Trunks from Cell's timeline would have killed the androids without going to another timeline. Besides, he makes very good points haha.

Furthermore, can someone answer me how many timelines there are at the end of the CGs and Boo saga [before Super] according to Dragon Ball Super? Like I said, iirc, I think there were 4 or 5 timerings and each timering features a timeline iirc. Now that I think there was a timeline many people consider that there wasn't -- a timeline from where the Mirai Trunks who killed Freeza in Cell's timeline came from -- it'd help if there were 5 timelines according to DBS, so that way I could consider this timeline asked in the OP as one that existed. I am trying to write it off because I was told long ago that according to Super there were just 4 timelines in the original manga, so if we consider this timeline the Trunks from Cell's timeline went to, then the timeline the Trunks that killed Freeza in Mirai Cell's timeline came from would have to be written off the story, and I don't like it.

So, dbgtFO, or someone else who don't see the need for the existence of such timeline, please clarify me that

[KBABZ]

What? How does the anime show Trunks killing the androids without a shutdown controller? I am not doubting you, just interested in seeing where that's implied as I haven't seen the anime in a long time.

Oops, I got confused there. I meant the Trunks who saw Gohan reach SS2 and destroy Cell. There's an episode in Kai (which is my knowledgebase, for the record) where Trunks returns to his time, catches Future Bulma up on how things went, Trunks goes and kills 17 and 18, then we see the narrator talk about Earth's recovery. After that we get the scene where Trunks confronts Cell.

My theory on the "unseen Timeline", the one that goes down before Cell shows up as we see in the show, is that Unseen Trunks and the other Z Fighters are able to kill the Androids. They were overpowered on the mountain road so defeating them would require a strength boost (the shutdown wouldn't work because the 18 kiss still happens so Krillin wouldn't do it). Ergo Unseen Trunks would then return to his future, kill the Androids there through strength, and then get killed by Cell because Unseen Trunks had no idea he was coming.

[ahill1]

Oops, I got confused there. I meant the Trunks who saw Gohan reach SS2 and destroy Cell. There's an episode in Kai (which is my knowledgebase, for the record) where Trunks returns to his time, catches Future Bulma up on how things went, Trunks goes and kills 17 and 18, then we see the narrator talk about Earth's recovery. After that we get the scene where Trunks confronts Cell.

My theory on the "unseen Timeline", the one that goes down before Cell shows up as we see in the show, is that Unseen Trunks and the other Z Fighters are able to kill the Androids. They were overpowered on the mountain road so defeating them would require a strength boost (the shutdown wouldn't work because the 18 kiss still happens so Krillin wouldn't do it). Ergo Unseen Trunks would then return to his future, kill the Androids there through strength, and then get killed by Cell because Unseen Trunks had no idea he was coming.

Oh, I see. There's this scene in the manga as well.

Some people think that the Trunks from Cell's timeline couldn't have killed the androids of that timeline with the shutdown controller as the blueprints were in the underground laboratory and Trunks just got the knowledge about the underground laboratory because Cell said that, to Piccolo... so, if Trunks didn't get to meet Cell, how would he know about the underground laboratory and, in consequence, the blueprints? But the problem is that Trunks was still powerless against the androids until training in the RoSaT, and even the Cell from this timeline we are discussing only sees Piccolo as a real fighter capable of fighting the androids... dunno how he might have killed them then.

[Desassina]

It's as simple as imagining that, when Trunks exists in the past, he must have come from another future, and because the other one exists, he can go to when he was present in that line's past. Otherwise, the time traveller will have created another, like Cell did.

[spoiler][/spoiler]

[ahill1]

It's as simple as imagining that, when Trunks exists in the past, he must have come from another future

So an infinite loop?

[Desassina]

More or less. It's more the case of not worrying about which line was the first, but to assume that they would be the same without interference, so that they are concurrent when they change.

[ahill1]

More or less. It's more the case of not worrying about which line was the first, but to assume that they would be the same without interference, so that they are concurrent when they change.

But there can't be an infinite number of timelines. According to Super, there were 4 or 5 timelines... the number is limited.

[Desassina]

You can chalk that off as the number of lines that were changed. An infinite number of lines that end up being the same should count as one. What would the point of keeping track of no difference at all be?

[ahill1]

You can chalk that off as the number of lines that were changed. An infinite number of lines that end up being the same should count as one. What would the point of keeping track of no difference at all be?

It's said the rings feature "different timelines", or something like this. Two timelines that are the same are still different timelines. So, if there were only 4/5 rings, we are then lead to think there were only this amount of timelines. I don't see the need to overcomplicate that.

[Desassina]

... Whatever.

[ahill1]

Err... I dunno. I am not saying I am right, but I saw Super specifying the number of timelines as a nice thing that could make our understanding on it easier. But you may be right, no way to know.

I was of the opinion that there was one timeline that was the trigger in creating the other ones.

[KBABZ]

Some people think that the Trunks from Cell's timeline couldn't have killed the androids of that timeline with the shutdown controller as the blueprints were in the underground laboratory and Trunks just got the knowledge about the underground laboratory because Cell said that, to Piccolo... so, if Trunks didn't get to meet Cell, how would he know about the underground laboratory and, in consequence, the blueprints? But the problem is that Trunks was still powerless against the androids until training in the RoSaT, and even the Cell from this timeline we are discussing only sees Piccolo as a real fighter capable of fighting the androids... dunno how he might have killed them then.

Ultimately we don't know exactly how the Cell-less encounter with the Androids the Z Fighters had went down, and can only speculate. We can assume that everything goes the same as what we see in the manga/anime up until Cell appears, at which point Piccolo decides merging with Kami is the only way to defeat them (Kami's premonitions about Cell were not the reason, at least in Z Kai dub). It's entirely possible that Trunks DID train in the RoSaT, but since it was only for the Androids and not Cell, he hit a middle-ground where he's able to defeat the comparatively less-powerful Androids in his future, but is still killed when Cell catches him by surprise later.

[Terra-jin]

Ultimately we don't know exactly how the Cell-less encounter with the Androids the Z Fighters had went down, and can only speculate.

We might be able to piece together the majority of events in the unseen encounter with the artificial humans.

• The timing of Goku’s illness is probably affected by Trunks’s interference, making it similar to main timeline.
  
  Goku’s survival in 766 must have created different circumstances for Dr. Gero. It’s reasonable to assume that Goku’s survival led Dr. Gero to preserve 16, improve 17 and 18, build 19 and transform himself into 20. This was probably Dr. Gero’s original plan, as well, before Goku’s death essentially eliminated the entire raison d'être for his revenge project, leading to the events in the future timelines.
  
  • Dr. Gero did state that he himself stopped spying on the Z-warriors after the battle with Vegeta, but his computer did continue (Cell pointed out a spy-robot to Piccolo). Furthermore, while he may not have been keeping tabs on their powers, he may have heard about Goku’s death through other channels.
  The secret laboratory basement remains hidden, because it was Cell who revealed its existence. Therefore Cell was never discovered and neither were the blueprints for 17 and 18.
  
  Trunks does acquire some means to stop the artificial humans in the original timeline, which implies they were stopped in the unseen timeline, as well.
  
  Cell kills Trunks, which implies Trunks was not strong enough and/or did not surpass SSj.
  
  Goku and Vegeta were the ones to reveal the possibility of surpassing SSj to Trunks in the main timeline. Without that, there’d be no incentive to use the RoSaT.
  
  Trunks planned to change history further in the year 763, which implies that things did not turn out optimally in the unseen timeline.

With these points, we can reconstruct to a good degree what happened in the original and unseen. Until 767, events unfolded nearly identically to the main timeline, with the only exception that Cell was not re-maturing in the ground, giving Kami an ominous, foreboding feeling.
The battle with 19 and 20 and the first battle with 17 and 18 went down exactly the same. Piccolo went to Kami, Vegeta flew off and the rest went to Goku’s house to hide him from the artificial humans while he recovers from the virus. After this, things diverge.
Kami does not yet merge with Piccolo, because he sees no definite threat in the artificial humans. Elsewhere, everyone gathers at Kame House with Goku, who eventually recovers. Even though he, and Vegeta, thought of surpassing SSj, Goku doesn’t yet resort to the RoSaT, like he did in the main timeline. He decides to meet the artificial humans head-on, aided by Trunks, Gohan, Tenshinhan, Krillin and Yamcha.
In the ensuing battle, Goku is killed at some point by 16’s self-destruction bomb. Gohan snaps and turns SSj, and Kami finally merges with Piccolo. Vegeta also arrives for his rematch against 18. Tenshinhan and Krillin help out in crucial moments with their special techniques (the kikoho, kienzan and tayouken) and together, the team manages to win against the artificial humans. They preserve 17 and 18’s bodies for Bulma to find a weakness from them. Vegeta once again flies off and silently, solitarily concentrates on finding a way to surpass SSj.
In the aftermath, Trunks never learned of the higher SSj forms, but Bulma did make a shutdown controller for him. He never learns of Cell. Since Goku died and the Dragonballs are gone, Trunks promises Bulma to change history further to create a fully peaceful future, instead of this pyrrhic victory, and departs for the original timeline. Little did they know that Goku would contact the Nameks from the afterlife to restore the Dragonballs and that he would come back to life during the Majin Buu arc through Old Kaioshin.
Meanwhile, Trunks had killed 17 and 18 in his future. Cell, who was absorbing people to slowly build his power, eventually finds out that 17 and 18 were killed. He finds Trunks and his time machine and kills him, traveling to the year 763 and well, the rest is history.

[KBABZ]

-insert Unseen Timeline Summary here-

That's pretty good! Feels like a natural flow to me. I have a couple of disagreements though. Here's how I think it went down:


- Trunks interacting with everyone and warning of the Androids shifts the time of Goku's heart virus to what we see in the show (I'll call it the Z Timeline for simplicity's sake).

- I also agree that Goku's survival impacts Dr. Gero's work. To take it further though, Goku is not just alive but is training as well, which Gero will compensate for when designing 17, 18 and Cell. This would technically mean that the Cell coming from Future Trunks' Timeline wasn't designed for this (which means it's a good job Trunks and Krillin killed the embryo in the Z Timeline, that Cell would have been even worse!). In any case, Trunks doesn't think anything of this experiment because he doesn't know what it is or what it will grow into, hence why he gets killed by Cell later.

- I personally assumed that Gero always intended to be made into an Android, even in Future Trunks' Timeline. My headcanon is that 19 and 20 did show up in Future Trunks' Timeline and Gero was forced to retreat just as in the show; Trunks shows up after he's already on the ropes after all. So to me how this event goes for Future Trunks' Timeline is that Gero retreats, awakens 17 and 18, gets killed, and then 17 and 18 kill everybody except Gohan. Since Bulma and Yajirobe, the only surviving witnesses, were far away, they had no idea that 19 and 20 even existed in the first place, hence why Trunks is caught off-guard when he returns to help out. (the only flaw there is the Gohan is also a witness, which I didn't think of until now, heh)

- Training in the RoSaT I think is an option warranted by the Androids because they still handily beat all of the Z Fighters. Piccolo merges with Kami (eventually, Piccolo would need to do more persuading here) and Goku and Gohan probably go in the RoSaT to train, after Vegeta does.

- I don't think 16 gets awakened at all, but I can't figure out why. Your version would make a good OVA though!

- I think that the secret laboratory is found in the Unseen Timeline. I think how it goes is that 17 and 18 are defeated, Trunks and Krillin (and whoever else) go to Gero's Lab but can't find the blueprints for the Androids, and their desperate search leads them to find the hidden basement, and the embryonic Cell, whom they kill like in the Z Timeline. Once he recovers the plans, Trunks takes them to Bulma (most likely Unseen Bulma because she'd have better resources than Future Bulma) who makes the shutdown device (and probably one with a better range than the bash job one from the Z Timeline).

- I don't think anything majorly wrong happens in the Unseen Timeline. I think this because in the Z Kai dub (again my only reference point here), Trunks is returning to the past not to fix something else, but simply to let everyone in the Unseen Timeline know that he was able to defeat the Androids in his future. I think Cell catches him here as he does in the "post-Z Future Trunks Timeline future" that we see in the show because Trunks had the time machine charged up and ready to go, and didn't want to risk figuring out how to start, prepare and operate it.[/list]

[Terra-jin]

- I personally assumed that Gero always intended to be made into an Android, even in Future Trunks' Timeline. My headcanon is that 19 and 20 did show up in Future Trunks' Timeline and Gero was forced to retreat just as in the show; Trunks shows up after he's already on the ropes after all. So to me how this event goes for Future Trunks' Timeline is that Gero retreats, awakens 17 and 18, gets killed, and then 17 and 18 kill everybody except Gohan. Since Bulma and Yajirobe, the only surviving witnesses, were far away, they had no idea that 19 and 20 even existed in the first place, hence why Trunks is caught off-guard when he returns to help out. (the only flaw there is the Gohan is also a witness, which I didn't think of until now, heh)

That's possible, yes. It makes sense that Dr. Gero would want to carry out or at least witness his sweet revenge against Goku.

Cell has an anime-only flashback in which 17 and 18 kill Dr. Gero while he's still in his lab and inadvertently leave Cell's incubation tank intact, which contradicts your theory, but it depends on whether you accept it as canon.

- Training in the RoSaT I think is an option warranted by the Androids because they still handily beat all of the Z Fighters. Piccolo merges with Kami (eventually, Piccolo would need to do more persuading here) and Goku and Gohan probably go in the RoSaT to train, after Vegeta does.

Admittedly, this is a weak point in my story. I'll chalk it up to Goku wanting to fight the artificial humans and not seeing them as a big enough threat to resort to the RoSaT. In the main timeline, semi-perfect Cell was much farther above Goku than 17 and 18, so he'd see no choice but to aim for the power beyond Super Saiyan. But the RoSaT would certainly be a prudent choice in the unseen timeline, as well. Only problem is that if the RoSaT were used, Trunks would probably have been there, as well, which would make him stronger than Cell... so in order to keep Trunks weaker than Cell, I'd say they didn't use it in the unseen timeline.

- I think that the secret laboratory is found in the Unseen Timeline. I think how it goes is that 17 and 18 are defeated, Trunks and Krillin (and whoever else) go to Gero's Lab but can't find the blueprints for the Androids, and their desperate search leads them to find the hidden basement, and the embryonic Cell, whom they kill like in the Z Timeline. Once he recovers the plans, Trunks takes them to Bulma (most likely Unseen Bulma because she'd have better resources than Future Bulma) who makes the shutdown device (and probably one with a better range than the bash job one from the Z Timeline).

The problem I see with that scenario is that nothing would have stopped Trunks and Future Gohan from rummaging the rubble in their own timeline. They could've found the plans there and there'd never even be a need for time travel.

This might work: when they were knocking at the door of Dr. Gero's lab, the Z-senshi got a look inside and may have seen the stairs to the basement. That might have prompted them to do a more thorough search of the wrecked lab than Trunks and Future Gohan did in the future. I mean, I'm sure that someone in the future though to look around in Dr. Gero's lab, though apparently they didn't find the basement back then.

- I don't think anything majorly wrong happens in the Unseen Timeline. I think this because in the Z Kai dub (again my only reference point here), Trunks is returning to the past not to fix something else, but simply to let everyone in the Unseen Timeline know that he was able to defeat the Androids in his future. I think Cell catches him here as he does in the "post-Z Future Trunks Timeline future" that we see in the show because Trunks had the time machine charged up and ready to go, and didn't want to risk figuring out how to start, prepare and operate it.

It does still seem strange that Trunks would set the destination year to 763 if he wanted to celebrate with his friends in the past. He should've set it to 767 if that was his purpose. It may be that he didn't finish setting up the time machine for his intended destination year, but one wonders why 763 pops up in the first place. Either Trunks had some reason to go specifically to that year, or Cell did in fact enter his preferred destination year.

[dbgtFO]

@dbgtFO --> only a doubt and I'd like to hear your opinion. How do you think Trunks [from Cell's timeline] killed the androids if he didn't travel to the past? He couldn't have possibly found out about the underground laboratory and get the blue prints since he wouldn't know about Cell.

Also, I'd like to see @dbgtFO's opinion on this. He, like me, doesn't see the need for the existence of this unseen timeline, so I'd like to see his opinion on how the Trunks from Cell's timeline would have killed the androids without going to another timeline. Besides, he makes very good points haha.

So, dbgtFO, or someone else who don't see the need for the existence of such timeline, please clarify me that

Not so sure, Trunks couldn't get the blueprints, even if there was no Cell to tell him about the existence of the underground basement.
We know for a fact, that main timeline Bulma recognized Dr. Gero on sight and knew he had a lab in the mountains. Cell was of course already in the main timeline, but I'm going out on a limb here and saying I really doubt that should change whether or not Bulma knew that between the two timelines.
So I assume she also knew about that in the second timeline(the timeline, that at the time of the Android's release only differed from the main timeline by not having Cell already there) and so here she and or Trunks puts that knowledge to use by eventually finding it after several years of searching.
Problem here is that they then failed to have the lab be completely destroyed and thus allows Cell to grow, which is of course a ridiculous plot contrivance.

A wacky thought I had was that she weaponized the hoi poi capsules and beat them that way, maybe caught them like Pokemon

But seriously, while it would be ridiculous, maybe Bulma eventually made one without the blueprints.
It's also possible #16 somehow destroyed #17 and #18 after being unhappy, that they were just destroying stuff.
Or maybe Trunks just got strong enough to beat them after years of training.
Either way I have to resort to quite some assumptions..

Furthermore, can someone answer me how many timelines there are at the end of the CGs and Boo saga [before Super] according to Dragon Ball Super? Like I said, iirc, I think there were 4 or 5 timerings and each timering features a timeline iirc. Now that I think there was a timeline many people consider that there wasn't -- a timeline from where the Mirai Trunks who killed Freeza in Cell's timeline came from -- it'd help if there were 5 timelines according to DBS, so that way I could consider this timeline asked in the OP as one that existed. I am trying to write it off because I was told long ago that according to Super there were just 4 timelines in the original manga, so if we consider this timeline the Trunks from Cell's timeline went to, then the timeline the Trunks that killed Freeza in Mirai Cell's timeline came from would have to be written off the story, and I don't like it.

Seems like there are supposed to be 5 timelines, before things started to change with Zamas' death and later the erasure of Future Trunks' timeline and then the creation of another.

[ahill1]

@dbgtFO --> only a doubt and I'd like to hear your opinion. How do you think Trunks [from Cell's timeline] killed the androids if he didn't travel to the past? He couldn't have possibly found out about the underground laboratory and get the blue prints since he wouldn't know about Cell.

Also, I'd like to see @dbgtFO's opinion on this. He, like me, doesn't see the need for the existence of this unseen timeline, so I'd like to see his opinion on how the Trunks from Cell's timeline would have killed the androids without going to another timeline. Besides, he makes very good points haha.

So, dbgtFO, or someone else who don't see the need for the existence of such timeline, please clarify me that

Not so sure, Trunks couldn't get the blueprints, even if there was no Cell to tell him about the existence of the underground basement.
We know for a fact, that main timeline Bulma recognized Dr. Gero on sight and knew he had a lab in the mountains. Cell was of course already in the main timeline, but I'm going out on a limb here and saying I really doubt that should change whether or not Bulma knew that between the two timelines.
So I assume she also knew about that in the second timeline(the timeline, that at the time of the Android's release only differed from the main timeline by not having Cell already there) and so here she and or Trunks puts that knowledge to use by eventually finding it after several years of searching.
Problem here is that they then failed to have the lab be completely destroyed and thus allows Cell to grow, which is of course a ridiculous plot contrivance.

A wacky thought I had was that she weaponized the hoi poi capsules and beat them that way, maybe caught them like Pokemon

But seriously, while it would be ridiculous, maybe Bulma eventually made one without the blueprints.
It's also possible #16 somehow destroyed #17 and #18 after being unhappy, that they were just destroying stuff.
Or maybe Trunks just got strong enough to beat them after years of training.
Either way I have to resort to quite some assumptions..

Furthermore, can someone answer me how many timelines there are at the end of the CGs and Boo saga [before Super] according to Dragon Ball Super? Like I said, iirc, I think there were 4 or 5 timerings and each timering features a timeline iirc. Now that I think there was a timeline many people consider that there wasn't -- a timeline from where the Mirai Trunks who killed Freeza in Cell's timeline came from -- it'd help if there were 5 timelines according to DBS, so that way I could consider this timeline asked in the OP as one that existed. I am trying to write it off because I was told long ago that according to Super there were just 4 timelines in the original manga, so if we consider this timeline the Trunks from Cell's timeline went to, then the timeline the Trunks that killed Freeza in Mirai Cell's timeline came from would have to be written off the story, and I don't like it.

Seems like there are supposed to be 5 timelines, before things started to change with Zamas' death and later the erasure of Future Trunks' timeline and then the creation of another.

I see. Thanks for your response. One thing I was thinking though, dbgtFO...

People assume that the Trunks in the main timeline had SSJ at a very young age (before even 8 or at the very least at 8 apparently) because Vegeta had sex witth Bulma when he was already a SSJ, so maybe using AT's recent interview he inherited more S-cells (can't remember that exactly). It's debatable whether Vegeta -- from future Trunks' timeline -- was a SSJ or not as far as the manga goes, so that could explain the difference in the time and mastery as well as potential of both the Trunks from the main and future timeline... but considering that in Cell's timeline Trunks advised everyone about the androids and that Goku was with the medicine, Vegeta should have pushed himself as far as he could go in that timeline as well and logically obtained the SSJ much like in the main timeline. So, shouldn't the Trunks from Cell's timeline have logically the same potential as the one from the main timeline? If so, that could explain how that Trunks could deal with the androids of that timeline with his strength... but then it doesn't explain how Cell killed him, apparently easily (whereas Trunks from the main timeline should have been able to make easy work out of that Cell) and how Cell stated how Kamiccolo was the only one who should have been a match for the androids. What do you think?


Also, if there were 5 timelines according to DBS, then we could incorporate this timeline asked in the OP as one that also existed, and therefore we'd end up with 5 and we wouldn't need to write this timeline or the timeline to which future Trunks who killed Freeza in Cell's timeline came from out of the story. But if we consider that the Trunks from Cell's timeline had the same potential and mastery over ssj as the one from the main timeline, then that poses a problem, since he should have been strong to deal with them, but at the same time he is weak enough to be done in by Cell, who should be at most on par with 16.... man, I dunno.

[KBABZ]

Cell has an anime-only flashback in which 17 and 18 kill Dr. Gero while he's still in his lab and inadvertently leave Cell's incubation tank intact, which contradicts your theory, but it depends on whether you accept it as canon.

You'll have to explain how this contradicts my theory?

The problem I see with that scenario is that nothing would have stopped Trunks and Future Gohan from rummaging the rubble in their own timeline. They could've found the plans there and there'd never even be a need for time travel.

Considering how much of a rampage 17 and 18 go on, we could assume that a deflected shot or a bad day caused them to blow up the entire mountain, or something like that.

It does still seem strange that Trunks would set the destination year to 763 if he wanted to celebrate with his friends in the past. He should've set it to 767 if that was his purpose. It may be that he didn't finish setting up the time machine for his intended destination year, but one wonders why 763 pops up in the first place. Either Trunks had some reason to go specifically to that year, or Cell did in fact enter his preferred destination year.

That's the biggest problem I see with where Cell comes from. If I recall correctly, Cell's personal flashback to this event has him kill Trunks in front of the Time Machine at night in the wilderness, whereas what we see later on is that Cell attempts to kill him in broad daylight in the middle of West City. The date is also confusing; I figured that maybe Trunks had set it there for whatever reason before takeoff (maybe a nostalgic look back on his adventures so far?), and intended to adjust the date once he got back in the Time Machine (but Cell kills him before he can do that).

[Terra-jin]

You'll have to explain how this contradicts my theory?

Hmmmm... I may have made a mistake here. The flashback shows Dr. Gero being killed in the lab, while he was working on Cell. He was already a cyborg at that point. I assumed he didn't yet go out and got beaten by the Z-senshi, but he might have (though under different circumstances).

Considering how much of a rampage 17 and 18 go on, we could assume that a deflected shot or a bad day caused them to blow up the entire mountain, or something like that.

This would've had to have happened after Cell was already born, otherwise Cell wouldn't have survived.

It does seem strange that 17 and 18 would leave the secret basement intact, even by chance... or was the basement, contrary to the aforementioned flashback, a secret even to 17 and 18?

That's the biggest problem I see with where Cell comes from. If I recall correctly, Cell's personal flashback to this event has him kill Trunks in front of the Time Machine at night in the wilderness, whereas what we see later on is that Cell attempts to kill him in broad daylight in the middle of West City. The date is also confusing; I figured that maybe Trunks had set it there for whatever reason before takeoff (maybe a nostalgic look back on his adventures so far?), and intended to adjust the date once he got back in the Time Machine (but Cell kills him before he can do that).

I personally like the idea better that he wanted to fix up the timeline some more. It intensifies the mysterious feel of the unseen timeline with the suspicion that things didn't turn out optimally.

On the difference between Cell's flashback and "our" Future Trunks encounter with Future Cell: it makes sense to me that these encounters took place under different circumstances. The two future timelines started diverging from roughly 784, after Trunks first returns from the past. The most notable change from the original timeline compared to Trunks' future timeline is that Trunks at some point killed 17 and 18 with a power greater than Cell's. Whereas Original Cell could still be confident about having the strongest power around, Future Cell had to contend with a power far outclassing his own. It stands to reason that their circumstances would shift.

As a side note: the date 788 applies to the moment Original Cell took Original Trunks' time machine. Most people assume that Future Trunks encounter with Future Cell also happened in 788, but this isn't necessarily the case. In my headcanon, Future Cell showed up some time later than when his Original counterpart did, hence the difference in time of day.

[KBABZ]

This would've had to have happened after Cell was already born, otherwise Cell wouldn't have survived.

Well yes, obviously!

It does seem strange that 17 and 18 would leave the secret basement intact, even by chance... or was the basement, contrary to the aforementioned flashback, a secret even to 17 and 18?

I think so and it makes sense. Firstly, the Androids blow up most the lab before they leave (at least in the Z Timeline), and Trunks and Krillin would of course return later to the secret basement lab. But second and more importantly, Dr. Gero made a creation whose power is dependent on absorbing the two delinquents. There's no way they would have agreed to that, hence Gero would obviously want to keep the lab where he created Cell hidden from them.

The most notable change from the original timeline compared to Trunks' future timeline is that Trunks at some point killed 17 and 18 with a power greater than Cell's.

For fact-checking's sake, do we know what method Cell used to kill Unseen Trunks?

As a side note: the date 788 applies to the moment Original Cell took Original Trunks' time machine. Most people assume that Future Trunks encounter with Future Cell also happened in 788, but this isn't necessarily the case. In my headcanon, Future Cell showed up some time later than when his Original counterpart did, hence the difference in time of day.

Sounds reasonable. I've always been a believer in micro-ripples, which is similar to the observation effect. Basically, something as seemingly insignificant as Trunks having a higher power level when returning to the future can cause Cell to delay (or hasten!) his confrontation with Trunks, simply due to minor mental reasons. Kinda similar to the For Want of a Nail trope.