I wouldn't call newly revealed information "retcon" but like... I don't see your point? It retroactively applies?
So then it's fine to use Elder Kai’s estimate of how long it’d take Beerus to wake up, a.k.a. Buutenks and Kid Buu would’ve destroyed the universe within 50 years tops.
Even in Super, Saiyans don’t passively grow stronger just by sitting on their ass or doing menial chores either, like they have to actually go out and fight comparable foes to get the super duper boosts, which Buu obviously wasn't going to find in his rampage.
Huh... no? If you do the same to Buu... he will receive the damage, then heal the Damage... he is not invulnerable
Cell still has to expend energy to regenerate and if he doesn't he just stays injured.
Buu can deform and pull his body parts off with no real harm to himself.
Even then, nothing indicates Cell got a Zenkai after Vegeta's Final Flash or Goku's Instant Kamehameha, so clearly something needs to almost kill him (He did say him surviving his Self-Destruct was a miracle) to even trigger it. Buu's regen is leagues better than Cell's, what's putting him at near-death lol
I mean... the fusion amp may just be that high i guess, not like he would be able to do that before they killed him for good anyway
Ultimate Gohan was weaker than x2 Super Buu, I really doubt Buu being able to spam SPC-esque Zenkais couldn't have breached that gap.
Not to mention, Elder Kai going "Hmmm, if I do a rough calculation based on how fast Saiyans get stronger over time (Something they're sooo know for we never even saw it pre-Super lol), Buu would exponentially grow strong enough to destroy the universe" is wayyy more of a reach than him just sensing Buutenks' power and going "Yup, we're doomed. He's destroying the universe."