Originally Posted by Michael J Fox is Canadian
There is no evidince it is not an alternate 2015 in fact the deleted scene is evidence that it IS. All that we see is hilldale.
If it is an alternate 2015 then Doc and Marty should be alternates too!
Just because we as the movie spectators stayed at 2015 and didn't follow Biff's trip to 1955 doesn't mean that suddenly there are different rules in the time-travel-theory which BTTF 1 has set up.
I mean Marty altered 1955 in the first movie and all people that "stayed" in 1985 and did not time-travel were of course affected by Marty's intervention. As the whole 1985 setting was.
But now in BTTF II when Biff does exactly the same Marty and Doc curiously remain the same and only the surroundings changed? Thats weird.
So if 2015 changed, Doc and Marty should too.