How could our heroes & old Biff switch timeline while keeping Almanac with young Biff?

Question

In Back to the Future 2, old Biff stole the time machine in 2015 and went back in time to give an Almanac to young Biff. At the point of giving the Almanac to young Biff in 1955, he altered the future making current reality flow in another timeline (with bad future & bad 2015). So, how could he return to good 2015 of the previous timeline?
Plus, when our heroes returned back to 1985, how could they go to the bad timeline?

Answer 1

Biff didn't return to the good 2015. It is true that we don't see any visible changes when old Biff returns, but it doesn't mean that the timeline is the same. The house where Marty and Jennnifer would live is likely to be inhabited by other people at the time when Biff arrives to 2015.

Also Biff feels extreme pain when he comes back because he changed the future to another timeline in which Biff no longer exists (Lorraine could have shot him somewhere in 90s). In a deleted scene we see what happens next: Biff fades like Marty in the first film and eventually dies. Zemeckis and Gale decided to cut this scene because they thought that it would be too confusing for the audience.

This explanation was given by Zemeckis and Gale in the bonus materials on my DVDs.

Answer 2

There are a number of theories around the time travel in the Back to the Future series, such as this from:

In order to account for these discrepancies without completely discarding Doc’s belief in traditional time travel theory, we have to assume that the “final” timeline is predestined.

For example, when Marty and Doc travel back in time and discover Biff’s alternate reality, this is a temporary tangent, not the “final” timeline. Ultimately, Marty does recover the almanac from young Biff and restores his original reality.

Even though Marty doesn’t save the day until the end of the movie, this certainly doesn’t mean that it hasn’t already happened in the “predestined” timeline. We don’t follow old Biff’s story once he returns to the future, but his plan could have already failed, assuming the predestined timeline prevails.

( http://www.overthinkingit.com/2009/01/16/how-time-travel-works-in-back-to-the-future/ )

Another discussion of time travel in Back to the Future Part II (and other movies) can be found at http://www.mjyoung.net/time/back2.html

Answer 3

It seems that changes in the timeline themselves take time to happen. That is, there is a sort of meta-time. Ryan North explains:

To buy into Back To The Future, you need to accept not only that time travel exists, but that there exists a META-TIME, because changes to the timeline THEMSELVES take time: Marty stops his parents from meeting and rather than disappearing right away, he has a week in 1955 to sort this out before the consequences of that become critical. In other words, whatever change you make to the timeline ripples through it like a wave in a bedsheet, altering things as it goes, and you’ve got until when that wave catches up with you to fix things if you’ve done something dumb like prevent yourself from being born.

Ryan posits that certain kinds of changes propagate more quickly through the timeline (i.e. when Marty's parents kiss at the dance, he is immediately restored to health). Even if the speed of the propagation is unpredictable, it still leaves the possibility open for someone to erase a future, and people in that future having time to notice and escape to the past to try to restore it.