The context here is he doesn't believe either fighter on their own can beat this Buu (which is why he suggested both and uses the words "fight")
Remember what I pointed out about over analyizing? Now if your statement was true and mine was false - Goku and Vegeta's comments about Buu coming back would be meaningless, Goku's comments to Goten in Dragonball Super would be meaningless and Gohan's look of worry on his face (when inquiring about Buu's death) would be meaningless.
Again, the entire conversation Goku and Vegeta were having at the time was the use of the Dragon Balls to set up a means for them to defeat Buu, so naturally any idea they would come up with would be means to that end. None of Goku's comments toward Goten in any way change that Gohan and Gotenks would still have been better off in beating Buu. Besides, again, why would Goku bring them up if he didn't believe they could beat him? His own body is ravaged and unable to even hold a charge as Super Saiya-jin 3, so there's no indication that he planned to just have them to be brought back to life just to stall him for awhile.
You being caught up on the whole "fight" vs. "beat" situation is basically like this. Say my friend and I are coming up with ways to raise money to buy a new video game system, and I say "Oh! I know! Let's sell some old comic books that we don't want anymore". According to you, because I'm not expressly saying "Let's sell some old comic books that we don't want anymore to raise money for a new video game system" then my reason for bringing it up isn't toward that goal. However, given the context of why I brought it up, it's clear what the reason behind it is.
So yes, the Spirit Bomb WAS the only hope, Goku goes as far to say that if Satan and Buu didn't help EVERYONE would have died.
And I explained how that line is relevant regardless of whether or not they had even used the Genki Dama in the first place. If Mr. Satan hadn't stepped in initially when he did, then Vegeta, who was trying to stall Buu for a minute for Goku to gather ki as Ssj3, would have been killed by Buu. In addition, his stepping in caused Mr. Buu to be spat out, who further stalled Pure Buu for awhile before Goku even fell out of Ssj3, unable to hold a charge. So there's a decent gap in time there where Mr. Buu (thanks to Mr. Satan) is stalling Pure Buu before Vegeta even brings up the Dragon Balls and restoring Earth's population, and if it hadn't been for Mr. Buu, Vegeta would have died/ceased to exist before the idea was brought up to use the Dragon Balls.
Even if Gohan and Gotenks had been brought up to defeat Buu and had done so, it would have still been thanks to Mr. Satan and Mr. Buu that Pure Buu was destroyed.
And no, the "planet buster" theory doesn't work, as Goku specifically talks about FIGHTING Buu again if that happens. Again, you're fabricating things not stated.
Except it's Vegeta that brought up the whole everyone would be doomed aspect, not Goku, and since he doesn't specify how everyone would be doomed, then the "planet buster" theory works perfectly well. It's Goku that specifies it being a situation of them needing to fight.
Vegeta: “…Don’t you get it!? What do you intend to do if he gives birth to that terrible Boo again!? This time for sure it might really mean the end of the world! It’s best to kill him now. Got that, you idiot?!”
See? He doesn't mention anything about fighting at all, so what he's potentially fearful for could readily still be a planet buster situation.
Honestly everything you stated, there isn't one statement in any media source to support it, be in manga, anime, interviews or daizenshuu.
That describes your situation more than mine. When you have the manga pretty clearly pointing to Pure Buu being weaker than Evil Buu, guide books supporting this by establishing that Buu lost power when he lost his absorptions (and even shows the scene of Mr. Buu's cocoon being ripped away to illustrate that fact), guide books saying that Gohan is stronger than Super Saiya-jin 3 (not specifying who either, so a blanket statement that would have to include Goku), etc, then yeah, there's more evidence pointing to Gohan being able to beat Pure Buu just fine.