The "BTTF" flicks were VERY CONTEMPTUOUS of maintaining temporal integrity insofar as the protagonists &/or their family, friends, etc were concerned. It was OK for the universe to be changed from the beginning of the movie, with Dad Wuss McFly being bullied by Bully Tannen, to the universe at the end of the movie, wiht Wuss Tannen being subservient to Dad McFly. Interesting how the beginning & end of the movie are mirror universes of each other insofar as McFly and Tannen are concerned.
Then in "BTTF II" it's OK for Doc Brown to change future history to benefity Marty's kid. And then again in "BTTF III" they let history be changed when Mary Steenburgen doesn't fall off that cliff & die. Plus other examples from III, just can't remember them now. So in the "BTTF" Trilogy we/you end up with what, 2, 3, 4, maybe more, parallel universes? It was BAD when Biff spawned that parallel universe where he was TEH $HIT. But mon contraire to the makers of "BTTF", it was also BAD for the protagonists or "heros", to change history for their own good. And/or if history being changed worked out to the good guys' benefit, so be it
So much for temporal integrity!