In BTTF 2, in the second timeline where Biff went back to 1955 to give the sports almanac to younger Biff, it doesn't make sense that there would be an original Marty running around.

This is my reasoning: Since in the original timeline, Doc Brown was just a local inventor in 1985, Marty was able to go back in time because Doc Brown showed him the time machine at night, which led to the terrorists killing the Doc and forcing Marty to go to 1955. In the altered timeline, however, the Doc was committed before 1985, which means that it's possible that he never managed to build a time machine at all, or even if he did, Marty wouldn't have gotten it. In this case, alternate-Marty wouldn't have been able to go back in time in the second timeline.

This is also a fact that Doc fails to reference when he's explaining the timelines. He forgets to talk about the original timeline - The 'lame parents' timeline which started the thing. Instead, everything should be fine exactly until November 5th, where the two timelines diverged. And so from November 5th-12th, there is the 'cool parents' timeline happening where Marty is back.

But the existence of the rich-Biff timeline negates the 'cool parents' timeline from existing at all.

This would imply that when normal-Marty went back to 1955 for the second time, he wouldn't have seen his own self because alternate-Marty never time travelled at all. Instead, the events would have played out exactly as they did in 1955 before Marty ever intervened, that is, George got hit by the van and the normal timeline would have been conserved. The only difference that should have been was that Old-Biff was giving Young-Biff the almanac.

Marty shouldn't have been able to see himself. Instead, he would have seen events play out exactly as they did in the 'lame-parents' timeline - George never learnt to be brave, there would be no drama about the enchantment under the sea dance.

Is this a plot hole, or is there some in-universe/retcon explanation for this?

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    I think the only answer is to take the advice given to another time traveller: youtube.com/watch?v=1SPk3NjYfmQ – Daniel Roseman Apr 3 '19 at 8:44
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    “it doesn't make sense that there would be an original Marty running around” — given that time travel is entirely fictional, it only doesn't make sense if you make up a set of rules that don't allow for it. The filmmakers don't contradict their own vague rules, so it's not a plot hole. – Paul D. Waite Apr 3 '19 at 8:50
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    @NathanKumar: so am I! If original Marty never meets Doc, he also never travels forward to 2015 and buys the Almanac, meaning old Biff never travels back to create the rich Biff timeline. So it seems that time travel in Back to the Future doesn't work in the way you're suggesting. – Paul D. Waite Apr 3 '19 at 8:57
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    But doesn't Marty II go back to 55 after Marty I arrives and before old Biff hands over the almanac therefor young Biff hasn't won any money, got Doc locked up and therefore nothing as yet has changed? – Seamusthedog Apr 3 '19 at 11:55
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    The very fact that Marty was slowly disappearing from the photo he had in the first movie tells us that changes to the timeline do not happen instantaneously. They never explicitly explain how it works, but there's clearly some kind of "ripple effect" that modifies the timeline slowly after someone makes a change. (What does that even mean, "changing the timeline slowly"? Don't look at me.) So, the second (first?) Marty was still there because he hadn't yet completely "vanished" due to the changes that should have removed him. – Steve-O Apr 3 '19 at 13:32