Can I ask a question about Part II totally unrelated to the OP?
I can? Cool.
Why would Doc and Marty have to go into the future to fix a mistake that hasn't happened yet? Once they travel back to 1985, wouldn't they have to fix that mistake again once the actual year rolls around? Or better yet, just tell Marty in 1985 what happens in 2015 that way the trip wasn't necessary?
I know why; because we wouldn't have a movie without it, but as far as I'm concerned, I'm cool with that!
Yeah, it's illogical but Zemeckis was pretty much "trapped" into doing a future story about Marty's kids because of the ending coda (written without any intent of doing a sequel.)
But in "reality" Doc could've just told Marty to raise his kids better/
not get in the accident that'll destroy his life.
The way BTTF's time-travel logic works Marty won't have to worry about the mistake "needing fixed again" even if his future continues to progress exactly the same, no more so than George reverted back to his passive ways after Marty left 1955. We
could reason that he couldn't simply tell his kid not do it later in the future because of the whole "kids don't listen to their parents" thing.
I think also recall from the novelization of the movie Marty Jr. wanting to talk to Marty Sr. (this is seen by Jeniffer as she's in the future M.McFly home) but Junior being shrugged off or simply told "later son" by his dad. Thus implying that if Marty was a more attentive father Marty Jr. wouldn't have been a part of the robbery. (Keep in mind the "real" Marty Jr. would have no idea what happened with Griff and likely still would've tried to meet-up with Griff, obviously wouldn't have been able to, and the robbery wouldn't take place.)
But all of this is moot because
now due to Marty not being in the accident his future is going to be very different and it's entirely possible Marty Junior (at least the one we meet in the movie) won't exist. And
no Marty won't become a rock-star. He
might become more active in other aspects of the music industry or simply just be a more aggressive or confident person less prone to pressure and being goaded into things but I do not accept that Marty's "destiny" or "future" is to become a rock star. If it is, he's better off with the 2015 future we see than one that leaves him an unintelligible old-man with collapsed veins and a death due to heroin abuse.