So, I was wondering if this universe had an afterlife, I mean I know about the whole reincarnation thing, but I’m sure some souls choose not to, and if so is there somewhere they could go, or would they just die? Also, if so, what if Mizar or someone else precious to Alcor decided not to return, would Alcor ever try to visit them or convince them to return, or has he read too many legends and myths where someone tries to go there and everything end up terribly for them?

There’s not a strict…afterlife, really, but there is a place where souls can go between incarnations. It’s closed off to any being not purely a soul, which means it’s inaccessible to Alcor. If a soul decided to stay in that other realm for any amount of time, he’d be unable to do anything about it. If Mizar did it, he would probably spiral a bit–even if he shouldn’t depend so much on her existence, he does it anyways.

In terms of length of stay in that other place, it varies. Souls typically don’t stay forever; some souls bounce back for a short while and then get out to go live life again, some souls take a bit longer, others take ages, and then maybe there are souls that stick around to only occasionally live. 

How often do you think Dipper would make a reference that no one gets and then instead of just explaining it, he actually shows them the original work? (translated, if they’re speaking a different language by that point in history)

He certainly tries, but even translated, some things don’t age well. Especially fantasy things.
(Also, Dipper has to end up translating the things himself, and a lot of the language ends up archaic or clunky because languages never translate directly all the time).

If I were a young teen in the Transcendence AU and I googled “Alcor the Dreambender” what kinds of articles and blogs and things would pop up?

It depends on when. TAU spans millenia. So does Alcor. Google…I’m not prepared to offer even an AU prediction on that.

If it’s way early on, you wouldn’t get many perfect matches before Google started returning things like “dream” on its own or stuff about the star or cyro companies with that name. Maybe there would be sites or blogs by people with a budding interest in the newly-discovered realm of demon summoning, listing known/discovered demons by names and titles. Lots of misinformation of course, and blatantly contradictory stuff. Maybe a news article or two if something really big happened and it was attributed to him, even tentatively. 

Go further, and you’ll have more demon enthusiast blogs and lists and first person accounts which may or may not be true. Possibly some memes, depending on what the collective consciousness of social media has grabbed onto in the meantime. More reports. Some PSA styled stuff about NOT summoning demons, especially these, and other posts by various individuals contradicting it with or without evidence. 

Get to and beyond the publishing and initial craze over Twin Souls, and good luck finding hits not related to that.

And on and on the rabbit hole goes, honestly. Fictional media, factually-styled stuff from various sources, memes and jokes and random chatter, occasional appearances of the name being used as a swear in some capacity, news reports, demonology research and publications, fanblogs, probably a thousand people online at any given time claiming to be the actual Dreambender (or just roleplaying)…