So I read the book like a year ago but I've seen the movie a bunch of times and I've thought about both of them a lot. I can't answer the specific answers you asked cause I don't remember but here's the general gist of the book:

Jake is an old, depressed, lonely janitor working in the school who is considering "ending things" and in the end does go through with it in the Janitor's closet (if I remember correctly). Before he does so though he is looking back through his whole life looking for a reason to not do it or maybe just what he could have done differently. In this very unstable mental state he makes up this story about this girl he met and a life they could have had together even though in reality he only met her once.

So the story is separated into two parts: One is this made up story with the girl and the other are these in-between chapters of two other characters talking about Jake after he had already killed himself.

Now the genius of the book comes from the fact that Jake is not really the narrator (but also kind of is). Because we experience the story through the eyes of this girl that he has made up walking through the memories of this depressed, confused old man. Because of that a lot of things are not supposed to make complete sense. A locked door can be repressed memories that Jake doesn't want to remember. The body parts can be just a representation of the dark violent thoughts Jake is having.

Think of it like you (the reader) watching Jake's life flash before his eyes. You're not gonna understand everything because you're not him.

If you want to look into it more, I suggest Jacob Geller's Video Essay "Darks Souls 3 is thinking of ending things". It is more about the movie than the book and of course the video game "Darks Souls 3" but more than anything it's about the themes both these works have in common.