The whole premise to Back to the Future 2 doesn't make any sense.
https://www.reddit.com/r/movies/comments/9rv218/the_whole_premise_to_back_to_the_future_2_doesnt/
When Doc comes back to the present to tell Marty that his son will be in trouble in the future, that is all that is necessary to prevent that from happening. 1985 Marty doesn't need to move forward in time to prevent the future event from happening, he can organically prevent it from happening when 2015 arrives. By going forward in time, he creates the unnecessary mess by clueing in future Biff.share
Too bad. It was the best movie of the franchise as a kid.