So, I just read through this (forgive me if this is kinda necroposting), but what if the divergeance point here is Goku & Gohan going into the time chamber first, rather than Vegeta & Trunks?
Assuming that they trained the same way in this universe as they did in the canon one, they didn't use the whole day. Which would make it possible that they got out of the time chamber either just before or just after Cell crashed the Piccolo/#17 fight (and with IT both of them could have feasibly got there in time to stop cell absorbing 17). I'd assume before and He, Piccolo & Gohan beat and presumably destroyed the androids before Cell got there.
Even if he didn't he'd have been more than strong enough to crush Semi-Perfect Cell. And while he's not averse to letitng an opponent power up so he can fight them at full potential, he's not Vegeta-level stupid about it. Gohan would probably talk him around - or take care of Cell himself.
What happens after that is uncertain. I imagine there'd be a rematch between FPSSJ Goku & Super Vegeta that Goku would win. Either that or Broly happens somewhere around here (I'm not sure how this chain of events would effect that story).
Either way I'm fairly sure that Gohan would, as in canon timeline, prioritize his studies over training - but with Goku around, wouldn't lose his edge. He'd have less raw power than his Buu saga counterpart, but more skill. Plus he's still on the edge of SSJ2, so has a lot of power. I'd imagine that Goku goes SSJ2 over the timeskip via training, although he doesn't show it off. Goten still gets concieved, but in this universe is trained by Goku and is stronger as a result. Vegeta keeps training to try catch up to Goku, pushing himself harder than in canon and by the time of the Buu Saga THINKS he's caught up.
No Hercule Satan - champion of Earth in this one, so no world martial arts tourney. They find out about Babidi & Buu in another way - presumably by Supreme Kai paying a visit to Dende.
I imagine they still do the Pui Pui/Yakon/Dabura fights, first 2 being the same result - Vegeta crushes Pui Pui, Goku reveals his SSJ2 to beat Yakon making Vegeta enraged. Gohan V Dabura would be different. Gohan doesn't have his SSJ2 power and is outclassed enough that he takes an injury and Goku has to step in as SSJ2. Dabura senses Vegeta's anger and envy of his full power and has Babidi majinize him. He assumes that he and Majin Vegeta will double team Goku, but Vegeta knocks him out and he and Goku have their SSJ2 fight, reviving Buu. Without SSJ2 and worn out from fighting each other, none of them is a match for Buu and they all die.
Dabura survives in this universe because he doesn't get to experience Buu's power first hand (which is what makes him realize he can't be controlled and attempt to kill him before he can turn on Babidi). Goku dies before planting the seed of rebellion in Buu's head. Buu kills everyone else, Babidi reigns.
That's probably not what happened. But it works (in my head at least).
