It suddenly occurs to me.....once Biff creates the alternate time line, not only should he not be able to return to original 2015, but original 2015 (where Doc and Marty are) should disappear completely and in alternate 1985 Doc and Marty should think everything is normal and be completely unaware that anything even happened.
This is by no means the only major plot hole (YOU can't just LEAVE Jennifer on a porch in alternate 1985! Wait. Can you?), but it seems the most egregious.
Eja, the movie's take on time travel is a little different. Change doesn't happen immediately. Hence in BTTF1 when Marty starts disappearing gradually. Technically, him meddling with his father's meeting of his mother should have instantly made him disappear, but then they wouldn't have much of a movie... so there's time to resolve the issue before Marty vanishes. There's been a bunch of essays, articles, wikis, etc dedicated to the BTTF timelines. Here's one: http://backtothefuture.wikia.com/wiki/Back_to_the_Future_timeline
But basically... Timeline 3, Doc and Marty travel to 2015. 2015 old man loser Biff goes back to 1955 to give his younger self an almanac... thus causing a ripple effect with a second timeline. When Doc and Marty travel back in time to 1985, you're right in that technically they should be immediately gone from existence. In-fact, when they arrive in 1985, they learn that Doc is dead. By the movie's time travel logic you can assume that they would both start to vanish from existence if they didn't immediately go back to 1955 to fix the splinter timeline. So they go backwards to 1955 to fix things.
Side note: I met Thomas Wilson (actor who played Biff) last year and saw him do some stand-up. He was surprisingly very talented. A song and dance man. Pretty versatile. You can tell he doesn't love being known for one thing... but it's amazing how much he looks like 2015 Biff.