Here's sort of the thing with how we're shown Time-Travel works in Back to the Future movies. As said above there is no "Earth 2", "3", etc. because we're shown that it's always the same timeline it just gets altered. In fact you can even watch these alterations take place, as we're shown throughout the movies on photographs and newspapers changing to reflect the new timeline.
(This is also one of the beautiful things about Biff and the almanac, as he gets richer and his power has its own impacts on things any alterations he may make to the world as a result of his wealth, and thus could impact sporting events, the almanac will change to reflect this as well. The almanac will always be useful, unless Biff ever made a change that will impact the existence of it.)
But there IS a problem with the end of Back to the Future, the original movie, with as we're shown how time travel works. We know from observation of the movies that there's no such thing as the "predestination paradox." The past CAN be changed, as well as the future. We're shown evidence that the 1985 Marty arrives in at the end of the first movie is now different from the 1985 he left. (The Lone Pine Mall sign.) This means that the Marty we see leave this 1985 doesn't have to leave in order for the changes to take place. They've already happened. They're not dependent on HIM. They're dependent on "our" Marty and cannot be altered. Theoretically that Marty can travel back in time to witness the new story of how his parents met and fell and love. Which at the end of the movie he does.
Which means he SHOULD travel back to 1955 and crash into himself.
"But, Trekker!", you say, "that's the point! He's supposed to go back there and make the changes!"
No, no he's not. Again, all because of how we're shown how time travel works in the movies. The changes are dependent on our primary Marty, the one we follow throughout the trilogy. Not that Marty. So, yes, there is a "second" Marty McFly somewhere and he never returns. We can probably assume, I guess, that the timeline is able to "correct" itself in order to avoid universe-ending paradoxes. Meaning when the "real" Marty of the altered 1985 leaves the "present" he just vanishes, consumed by time. Possibly displaced by instantly appearing at the exact same time and place as "our" Marty. Since they can't appear at the exact same time one gets pushed out of the way into lord knows where and when. Which might make for an interesting story, actually.
So we have "our" Marty who we see arrive on the Twin Pines Ranch in 1955 and makes the changes he does, at that exact same instant a point in space
we have the end of the movie Marty arriving at the ranch in 1955. They both can't appear at the very same time and place so one simply gets displaced. (I'm operating under the theory there has to be a "connection" between two time periods in order for time machines to work. Whether cosmic-strings or some other crazy thing, there needs to be something the time machine uses to travel between points in time.)
The time machine in the displacement suffers irreparable damage and arrives at some greatly distant time in the past (since that was the "direction" it was traveling.) And because we have to be SURE that things will unfold the way they "should" it's the better-off Marty that is displaced. So "our" Marty goes back thinking his father is a schmuck and knows the original story of how his parents met. The actual Marty of the altered-1985 would have heard a different story and may not have been as motivated to make the changes he needs to believing it's someone else who'll take care of, even more so considering his parents seemed to credit Biff with their meeting. (Oh, Biff, and his near-rape! Ha-ha!)
What time should he arrive in? What adventures would he have and deal with stuck in a time-period without Doc to repair the DeLorean.