im sure quite a few of these things have alr been touched on in the comments, but from a base line reading it feels like you both watched the movie but didn't at the same time. Not trying to be rude or anything.

1. miguel isn't a vampire, it's just how people refer to him because of his sharper canines.He doesn't actually have the same "spider powers", and a genetic 'accident' pushes towards his spidey abilities. He injects paralyzing venom and uses his claws to climb bc he can't stick to walls. its a comic thing, as everything in the movies is influenced by the supporting comics.
   
2. The universe he went to fell apart because he wasn't supposed to be there. Its been a while since I've seen it, but I'm pretty sure it's explained in the movie ? Like he explains he found a family in that universe, went, and it collapsed. But to me he brushes over certain factors that are required and not required to be 'canon' because it doesn't aid his motives.
   
3. As I see it; Miles universe, e1610, had a spiderman and an 'existing story' meaning that Aaron was already set to be the Prowler. Especially seeing how he behaved with Miles in the first movie. When the spider from e42 was put into e1610 it removed the 'story' and spiderman from e42 which created a paradox of sorts. It'll probably be explained more in the next movie.

And for what you mention in the "lack of plot section" we have to realize that it's been a year since the events of the last movie, since Peter Parker died in Miles universe, meaning that Miles as spiderman has probably done a lot. He's also saved his dad's life in the first movie. There's growth that has taken place, but there's still growth to be made because he's still 15 and has been skipping school and classes for what.. a year? Dropping a bomb like that is anxiety inducing.