Dragonwyck is one of my favourite movies of the 40s, and everyone I talk to has never even hear of it, so it's cool to find this discussion here! Just love Price's performance in this one! One of his best.
I actually read the book that this movie was based on this summer, and it answers some of your questions. For one, seeing the movie it's weird that Katrine is never seen again after her mother dies, but in the book she is sent away to live with an aunt.
I highly recommend checking out the book, it was actually much more exciting than the movie, and of course there's a lot more that happens (for example, Nicholas gets to meet his favourite writer, Edgar Allen Poe!).
But I really love watching this movie. Vincent Price is so hot!!