My back to the Future fan theory

Most of you know that Marty McFly was originally being played by another actor and that a lot of bttf scenes were already filmed with another guy in the leading role. However this all ties into the movie universe, in part two we find out about branching realities that are happening because doc and Marty are changing the past. Now this all takes a very dark twist compared to the tone of the originals which are light hearted adventure films. Every time doc used his time machine he created another timeline and his machine only travels back and forth in the timeline he created but the original timeline remains the same. Similar to Rick and Morty this makes the adventures of doc and Marty meaningless as there are infinite realities where Biff wins and those where they win. First time doc used the time machine he used it with original actor Marty but because they changed the past it caused current Marty at the beginning of bttf to change into another actor because his ancestry altered. This Marty watched the doc die and took his machine for a ride. And even though at the end the doc survives but that is not the same doc that died in the beginning. It is alternate timeline doc that met Marty in past, and we know it is the alternative future because Marty parents and family are again different. Therefore the real doc still died but that timeline is gone for Marty because the docs creation only moves between timelines it creates and not between parallel timelines like Rick Sanchez gun.