I had never really considered the ramifications of Original Marty coming back to the present and seeing new-timeline Marty going into the past...
http://www.wolfgnards.com/index.php/2013/05/12/back-to-back-to-the-future-a-tale-of-two-martys
But did he see new timeline Marty go to the past? Consider in BTTF2 when Old Biff time travels to give his younger self the Sports Almanac & then returns to the future he doesn't go to the new timeline - he returns to his old one.
Clearly any changes to the timeline aren't immediate upon returning to your original time - either the changes fade in over time (much like Marty was gradually fading out of existence in 1955 after preventing his own birth), or they require some further trigger to activate (like say the Delorean time travelling again - Old Biff's timeline doesn't change to Rich Biff's until after the Delorean is used again - Marty returns to his timeline, sees himself going back in time and
then the timeline changes).
Of course this brings with it new problems - what happened to new Marty? He didn't time travel. So does he exist separately from original Marty in the current timeline? And what happens when they meet?
Or did he just blip out of existence when the timeline changed (or rather not come into existence when everything else changed)? And was he alone when that happened? If not, what is that other person's reaction going to be to original Marty? They saw their Marty disappear & now there's this other person that looks like Marty & claims to be Marty walking around but their memories aren't quite right. Clearly they're not actually Marty! Who are they? Where did they come from? What are they up to?
And if new Marty does cease to exist in the new timeline does that mean original Marty essentially killed him? (Guns don't kill people...time travel kills people)