I have concrete answers for some of those questions and not others. With the burning book ending, the flame attack caused the book they were in to catch fire. With the framed portrait, that's an alcoholic beverage he's holding, which hints at the potential origin of his "monstrous" nature. I certainly think this could have benefited from being longer, and supporting those ideas with extra details along the way. Even exploring the implications of what it means to live inside a picture book: torn pages you can't cross, walking into pages from other books, etc.
EDIT: Also I'm glad you mentioned the music progressing even when you return to the start, I wanted to make it clear somehow that it wasn't a complete reset and that the story had progressed in some way.