MW03 said:
(1) In 1985, Adult Biff is an asshat.
(2) Marty goes to 1955, changes past.
(3) Marty returns to 1985, Adult Biff is neutered.
(4) 2015 Doc goes to 1985, collects Marty and Jennifer. They go to 2015.
(5) In 2015, Old Biff steals DeLorean.
(6) Old Biff goes to 1955, gives Almanac to Young Biff, changing the past.
(7) Old Biff then somehow returns to the same 2015 to stash the Delorean for Marty and Doc.
(8) Marty goes home to 1985, except now it's bad 1985.
Old Biff should have never been able to return to the same 2015 where he stole the DeLorean, just like how Marty couldn't return to the same 1985 with Asshat Biff. And we know it's not the same 2015, because George McFly is dead in Bad 1985.
I like debating this one.
Realize that in BTTF 1 when Marty goes back in time and knocks his father out of the road he doesn't immediately disappear, it takes a week of fading to remove his brothers hair and then his head and almost to Marty
The movie presents the idea that it isn't instantaneous.
now to number 7 and 8 on your list.
When Biff returns to 2015 things DO start to change just not instantaneous. Biff looks bad and seems to be ready to die, trash seems to blow in, and the weather changes. This means things are starting to change but haven't fully, just like the picture fading.
Meaning if Marty had stayed in 2015 it would have eventually changed all around him, but that's not what happen. He got in the DeLorean and went back to the "correct" changed 1985. It makes sense, "from a certain point of view."