The Wrong Door

I’ve just started the first chapter of The Magician’s Nephew from The Chronicles of Narnia.
My prediction is that the story is going to be like The Lion, the witch and the wardrobe but cut in half. Instead of two boys and two girls, it’s going to be one boy and one girl. I think that because the book is called The Magician’s Nephew and a nephew is a son of a mother or father’s sister or brother and the boy in the story will need help if he goes to the land of Narnia.
I read through the first chapter and found out that my prediction was sort of correct. It was about a boy with a help of a girl, but they didn’t get to Narnia yet. Instead they got into a tunnel in the girl’s house (Polly) and accidentally found themselves in the boy’s house (Digory). That was his uncle’s study room.
I have three questions. What is Digory’s uncle’s job? I think he might be a librarian because he has a lot of books and also my grandmother is a librarian.
Do I think Digory’s uncle is a bad character? Yes because he shut the door were the children appeared and now they are all locked.
My third question is: Why did Polly disappear when she touched a ring? Maybe she disappeared
because she might have crossed the portal into Narnia and she didn’t notice.

I have problems with the word grubby. I thought it meant stinky and dirty. I thought that because I reread the sentence and the evidence [clues] were ‘’It would have been grubbier if he had first rubbed his hands and then cried and then dried his face with his hands’’.