I have kept this to myself for about a year, mostly because the semantics necessary to describe this are beyond me...but here it goes.
From movie 2 onwards, there is an atrocious paradox. This is because of the trip to 2015, and the subsequent events that was caused by this. Basically, we have the following events happen in sequence:
-Doc and Marty go to 2015 where Future Marty was in a car accident.
-Old Biff goes to 1955, Doc and Marty follow and get the Almanac back,
-Lightning sends the Delorean to the old West, Marty follows.
-Marty overcomes his "chicken" phobia in the West and goes back to 1985
-Marty with new experiences does not get in a car accident.
This means that the 2015 from movie 2 is erased and Future Marty is no longer a loser. This means Marty Jr. will grow up better and there is no need for Doc to bring Marty to 2015. Biff never sees the almanac/time machine and he doesn't go to 1955 to meet his younger self. This means Marty doesn't go to the Wild West and never gets over his "chicken phobia", but this means that the 2015 with loser Future Marty DOES exist...
So, this is a complete paradox, as bad as there being a Carl Sagan in FCB's timeline.