Well when Marty returns to 1985 in the first movie, he'd be a completely different person (personality wise). He was raised originally by his whimpy, failure father but after changing his father into a confident, successful author, his upbringing would have been totally different. He might not even exist considering his parents might not have conceived him at the same time.
There are tons of plot holes and Ginsu is right. You can't pick things apart too much. Just enjoy the movies.