2

When Biff returns to 2015A, he disappears probably because Lorraine shot him in the 1996A or because in fact, he doesn't match with the Biff-A of 2015A so the original old Biff no longer exists and fade out of existence. Is it a paradox? I mean:

  1. If there isn't a Biff-A in 2015A because he died in 1996, nobody could have stolen the DeLorean and give the young Biff the book.

  2. If there's actually exists a Biff-A, he doesn't have to stole the car and give himself the book in 1955. So, again, nobody would have stolen the car and the young Biff would not have got the book.

It's like if Marty has changed the Doc mind in 1955 and the scientist would not have created the time machine in 1985, it would be a paradox because Marty could not have travelled in time.

3

In Back to the Future Part II, how could old Biff have made it back to 2015? Answers most of your question, but the other part is simply that changes to the time stream in BTTF are not instant. The characters have enough time to affect the changes before they are erased from existence. Paradox averted.

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