Draw a door part

I don't think she'd have been allowed to make it. Remember Juno and whatever other higher powers there are witness everything. That's why she knows Otto took the book even though she wasn't present. And it's obviously the higher powers who make the book appear in the first place for the dead to read. So I would imagine the door would've just slammed shut if Lydia got into the attic whether Adam and Barbara were through it or not.

That's something that puzzled me. How is it anything the ghosts could interact with, the Living could too? It's one thing if Lydia could see ghosts, but why would she be able to see the handbook, pick it up, or even be able to read it? How is it she could see the door to the Netherworld?

The rules regarding ghosts were very murky in this film. They hinted in the cartoon that the Living could go to the Netherworld, but only if they had magic.

They also don't make it clear how Beetlejuice, who is supposedly a ghost too, is able to have magical powers, shapeshift, and interact with both the spirit world and the living one, when none of the other ghosts could do that. There's a lot unanswered questions this movie (and no doubt the sequel) presented.