Originally Posted by Strayth
So, how is it even possible that Biff went to the past, gave himself the almanac (which creates the new timeline, "hell") and then COMES BACK to the original future ?
Whatever ramifications caused by Biff's actions that spilled into the year 2015 wouldn't necessarily be noticeable, at least not in such a way that a movie with BTTF's tone and running time restrictions could have realistically conveyed to the audience. And even if Biff giving the almanac to himself in 1985 did make Hill Valley 2015 a significantly different place visually than it was when Marty and Doc first saw it, they would never have noticed it because the timeline would have changed around them and they would only have memory of the new timeline (as Doc explains when Marty protests leaving Jennifer and Einstein behind when they realize they have to go back to 1955). Obviously, it doesn't all add up under close scrutiny, but it works in a movie logic kind of way.
The movie did address the alteration of the 2015 timeline in a deleted scene
where Biff fades out of existence. In the final cut, Biff is seen collapsing in pain shortly after returning to 2015 in the Delorean. Originally, this went on a bit longer, showing Biff literally disappearing the same way Marty almost did in BTTF1. The reason was because he'd returned to a future where he didn't exist anymore, because in the timeline where he is rich and powerful, he ends up getting shot (as speculated by Bob Gale) by Lorraine in the 90s.