https://screenrant.com/back-future-movies-original-alternate-timelines-explained/
Back to the Future's classic time travel trilogy has been delighting viewers for decades. Here's a breakdown of all eight timelines in the movies.
RE: The Fifth Timeline – Alternate/Dystopian 1985
As I understand, BTTF II has some troubling "paradoxes", or the Bob's error(s) being too lazy by saying it's "just a movie" [i.e., regarding Biff returning to 2015 in the Delorean from an alternate 1955].
But what always stood out to me, bothered me for so long but I didn't understand why, until now: in this alternate/"tangent" 1985, did Doc ever convert the Delorean into a time machine and request Marty to help him causing Marty to travel back to 1955 when Marty [i.e., the little SHIT, Biff] is supposed to be in Switzerland and Doc being committed to an insane asylum?
This question could be answered by stating the Doc's Western Union Letter, tombstone,... But that may only bring up more complications/paradoxes.
Or did alt' Doc invent the time machine and alt' Marty traveled back in time to 1955, as Marty was still in Hill Valley, unknowingly to alt' Biff's knowledge, and then on the day of or anytime after October 26, 1985, the Doc is committed to an insane asylum and alt' Marty is wondering around Hill Valley?
I can't remember if there was a date visible in the newspaper showcasing Doc's article. Is my "Or" situation what the audience is supposed to gather and understand what has happened in this alternate 1985?
I never thought about your third point! That's a big plot hole. Could be explained by this: (?) The events that have already occurred can't be altered since they occurred before the time change? I don't know, seems like weak logic to me, but it's all I can think of and it could work.
sharePer https://backtothefuture.fandom.com/wiki/Mental_Ward_B
Doc was committed to an insane asylum, coincidently, on May 23, 1983, a full forty years to this day (May 23, 2023)! This is about 3 years before 'our' Doc introduced the time machine to 'our' Marty on October 26, 1985.
I really don't what to make of it, there are way too many if/then and Ors that have run through my mind, making it difficult for me to head cannon any of this to make sure it fits the trilogies many "boxes".
I've tried to accept the 2015 return of Biff in the Delorean traveling from a past to a future, that doesn't exist anymore because of Biff altering everything with the use of the Sports Almanac, as a "delay" in the "ripple" or wave of time changing -- to maintain a one-timeline universe, but sadly it doesn't work for me anymore. I've been a fan of these films almost my whole life...
I have way more thoughts on my mind about this but it's too difficult for me to articulate at the moment. haha
EDIT:
All a really can believe now is that the Back to the Future universe utilizes multiple timelines, and not one, single one where "ripples in time" make changes to maintain a "one-timeline" universe; contrasting with the majority thought has been, for a long as I can remember [which I prescribed to], the BTTF universe occurred in a "one-timeline" universe.
Hm, yeah growing up as a big fan would make you know a lot. I also used to be a huge fan. I still love them, they're great. I never paid attention as much to the plot holes, though since I was big on the storytelling that was an important part of it.
I never thought about that part either, I suppose that would be an explanation. I mean, us as viewers would have to still see the future as it is, it wouldn't be able to just suddenly change while a past Marty and Doc are there (it would probably be the same while they're there and then change after they leave.) We see Biff die because of him altering the past, but that's the only change. They did leave pretty quickly after that. Idk it is very confusing.