Hi there!
Thanks for playing the game, happy that you enjoyed it, and thanks also for your question(s). :) You're definitely not dumb!
Yes, there are a lot of things going on with the plot in those final parts where everything is revealed! Everything you need to know is in the game, but some things are only hinted at so you'll actually have to piece some parts of it together yourself to get the whole picture. I'm the type of player myself who likes to do that which is why parts of the story is a bit open to interpretation, and you can't be 100% sure which character is telling the truth.
So my first recommendation would actually be to play it through again and see if you can figure everything out! But if you don't want to do that (which I totally understand), here come the answers below. And thank you again for playing and reaching out! :) <3
- Major spoiler answers to the above questions below! -
God: The Guide mentions another deity who is similar to what we humans would call God, though this thing isn't exactly the same thing as that. Guide sees it as a problem that this God has all the power and is fearful of what it could potentially do with it (as well as probably quite a bit jealous of him not having that power). "God" gets energy from humans in Purgatory; when people deal with the emotions that originally put them there, energy is produced that gets fed directly to "God", making it more powerful. This is how the system of Purgatory, and the game's afterlife, works, and how power is contained by the deities in question.
Items: The Guide has devised a way to contain the above mentioned energy in items, which he has Astrid use on unsuspecting people in Purgatory, proving that it works. Energy from emotions still has to be extracted manually, so he can't just go around stealing it from people. They have to willingly "open up", which is something that Astrid is really good at making people do. As they open up to her in The Guide route, their emotions of distress are channeled into the items, thus ridding them of their negative emotions, making them happy. This is a "cheat" that The Guide has come up with to be able to steal energy from "God".
Café: This is another place, i.e. it's not Purgatory. When in Purgatory, The Guide is constantly busy since he's simultaneously talking to millions of people at once, as he's hinted at before (him being omnipresent). He moves him and Astrid to the café setting, which is "another place", so he can be alone with his thoughts, and her.
Dance: As Astrid says, she pretty much gives in to all the absurd things that's happening and goes "ah well, might as well dance". In that moment she probably believes The Guide too when he says that he's trying to help in his own way, so she feels for him.
Dead boyfriend/Remm etc: This is part of the "Epilogue" in which you play as Vol, not Astrid. This happens at roughly the same time as Astrid's story, but somewhere else (not Purgatory). It's revealed when you get to the café and meet Remm that this place is actually the same café which The Guide took Astrid to, so this is in other words the domain of "God", and "God" is Remm (which is why she has those white eyes). The symbology is that Vol/Remm has a similar dynamic between them as Astrid/Guide, where one is a dead human and the other a deity which grows to like the human in question. So it's like a mirrored situation in a way.
As for why he's moving in with Astrid, this epilogue takes place in Vol's afterlife, in which he chooses to re-live that day over and over, because that's the day he realized that he loved Astrid. So that scene of them moving in together happened before they died.
Astrid in the end: Astrid decides to join The Guide, feeling like it's the lesser of two evils basically. This way she has a chance to help people get out of Purgatory faster instead of being miserable for hundreds of years (like some people end up doing), and she believes The Guide in that it's not a good thing that "God" has all the power. Time will tell if she's right or not. The reason why this pops up in the end (after the epilogue) is just that I felt it was more dramatic to show it there instead of after The Guide route, so you'd have to wait for a bit for the cliffhanger to resolve.
Phew! I hope that makes things at least a bit more clear. Let me know if there's anything else. :)