Look at things differently. Since Trunks is the only thing that connects these time lines, we will look at things from his point of view. That also, incidently, becomes the easiest way to look at things.
Firstly, let us consider Trunks' original timeline to be the Original Timeline, due to time travel not being around to change anything.
1) Gohan is killed by the Androids that killed everyone he knew three years before.
2) Trunks goes back in time to warn Goku, et al, then goes back to his future.
3) Trunks comes back to help everyone fight the Androids.
4) Trunks goes back to his time and Kills the Androids.
5) Trunks is killed by Cell.
6) Cell jumps to Our Timeline. Why? There is a very simple explanation below.
*Important, there have already been three jumps into the past, leaving plenty of opportunity for things to change the timeline we know from the Original Timeline..
7) Trunks learns about Cell and figures out his plan.

Everything we've come to know as the part of the Dragon Ball story happens.
9) Trunks jumpsback to the Original Timeline.
10) Trunks kills the Androids.
11) Trunks kills Cell.
12) The Altered Timeline appears.
There, in twelve steps is the story, in a Trunkscentric Dragon Universe. If the story were about him, that is the order the episodes would be in.
Now, we'll look at the Original Timeline as being supported by the timeline to which Trunks jumps back the first two times. By pure definition, that makes the second timeline nothing more than an ancillary time line, and that is why I am calling it the Ancillary Timeline. Nothing before Cell is outside of these two timelines.
Now, in Cell's pesonal timeline, we know the events of the Original Timeline and the Ancillary Timeline have already taken place. There is no other way that Trunks could have defeated the Androids. When Cell jumps back, he goes to another time line time line that hasn't happened yet, a secondary auxillary time line. Which, coincidently, is the time line we all know and love, Our Timeline.
After the Cell ordeal is over, Trunks goes back to the Original Timeline and the major events stay pretty much the same. Right up to the point where Cell plots to go back in time. At that point, Trunks destroys Cell. However, the events of Original Timeline's Trunks were a part of Cell's Past, and Cell was a part of Our Timeline's Future Trunks past. Cell's failure to kill Trunks forces the existence of another time line, and is the only altered future for the Original Timeline. That's why this one is called the Altered Timeline.
And now finally, we will take what we know, and remove the principality of Trunks from the equation. Placing the timelines parallel to each other we can see:
1) There is the Original Time that is supported by the Ancillary Timeline.
2) The Original Timeline Branches into the Altered Timeline.
3) The Altered Timeline could not exist without the prior existence of Our Timeline.
To sum up, it can be argued that there is only one main time line that with the help of the events of two supporting time lines, branches into a second time line. Or, it can be argued that there are four seperate interlinked time lines.
Both answers are correct since they are supported by evidence, and can not be proven wrong. And as as silly as that last sentence sounds, that is the foundation that EVERY scientific principal that we know is based on.
I'll be glad to answer any question, or clear up any point. But remember, although I think this makes the most sense, and it fits within all boundries that define scientific knowledge, it is only my theory. I am not claiming this to be set in stone fact.
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