Then back to the Future part 2, Marty McFly should have arrived in the future where he disappeared 30 years ago and his children were never born.
Even if he did arrive history should have begun reverting itself, as his disappearance from the past should have altered the present until he returns.
As long as he experienced no ghosting effects, that would have meant that he was functionally immortal until he returned back to the present.
That entire scenario could have been avoided if doc Brown had said we've got a few hours until the universe begins to rectify the fact that you are not in the past with the temporal causality of the present future