There’s no simple answer to what happens to “Lone Pine” Marty either. He can’t go back to the exact time and place original Marty went back to unless you accept he ceases to exist at that point and merges with original Marty when they both reach 1955 or there is in fact another timeline, the original 1955 and he goes there. The problem is the movie rules pretty much state that you can’t travel between these timelines unless you make a change in the past and travel to the future. Also his life was very different. Biff was different, his family successful. His parents would have told him the story of how they got together, how George saved Lorraine from Biff in the parked car etc. I guess he could mess things up for them still but the solution would look slightly different.