Maybe it's multiple timelines splitting off from decision points but once Marty involves himself in a particular timeline it becomes a part of his destiny. There have to be multiple timelines, otherwise how can there be multiple Martys?

As far as the fate of Doc Brown, it doesn't matter, does it? Because Marty will never experience that particular timeline again. It was altered.

I don't think Marty gets updated memories when the timeline is altered. What I don't understand is ok. In Back to the Future it seems as if time traveling Marty replaces himself in the timeline. So when he goes back to the Future, he does see himself escaping from the Libyans. But where does that Marty, the second Marty who escapes in the Future where Doc Brown lives, where does he go?!