Today's Darwin Award nominee

1) Maybe since Old Biff returned to 2015 before young Biff placed his first bet he came back to the correct timeline. In theory, Young Biff could have still ignored Old Biff at that point and kept the original timeline on course.

2) It was named Clayton Ravine in the original timeline Marty knows because Doc had never met Clara there. Once Doc goes back, however, he's always destined to meet and save Clara and from that point on it's always been Eastwood Ravine. Of course, you come back to whether all of this already happened and it should have always been Eastwood Ravine anyway and whether Hill Valley is now stuck in an irreversible time loop thanks to Doc and Marty.
Fuck, time travel is hard.

 

Another question ... When he's trying to escape the Libyans, Marty says, "Let's see if these bastards can do 90," obviously not thinking that going that fast will trigger the flux capacitor into doing its thing.
Where the hell is Marty going in a relatively closed parking lot that going 90 mph is going to allow him to escape the Libyans? Wouldn't he just run out of real estate faster and have to turn around? I know it's a mall, but how big and open is that parking lot that he's not hitting any light poles or concrete space dividers or medians going that fast?