Well, I love the movie trilogy and I try not to overthink these things. But, for the longest time, there is just one thing that has always been bugging me.
In Back to the Puture Part II, Marty & Doc go back to an alternate 1985 where Biff runs the town. We know Doc was sent to an Insane Asylum, but hey..."What happened to / Where is the Alternate '85 Marty during the movie??
Is he in jail? Is he dead? Or (my theory), did Biff have his goons send him away to another place, (LA maybe?) and is suprised that this Marty is back in town? I don't know.