A couple of questions..

I watched it the same way as you and had the same questions. My 'guess' is that:

Catherine was always dead because she was renting the house again at the end of the movie. That's also why the writer can leave without telling the police that he just killed Catherine. She was never alive. How she might've died though would then be the question to ask. Or you can say that because he never told the police about Cassie the whole scenario will play itself out again until her body is finally found. This of course then leaves the question of the white light and her (supposed) ascension.

None of that answers the question about his dreams which, from my perspective, had nothing to do with anything. I didn't think the movie was bad; it was kind of spooky and the acting was good. The script was just not anything original and pretty easy to guess even for someone watching with one eye.

http://twitter.com/AManAndAMouse/

everything between the music clip and him walking out of the door with a manuscript is fiction, the plot is a story within a story,

he rented a house to be alone, but the realtor, tricked him into sharing the house, with the old blind lady. so he wrote a ghost story and used them as evil characters, and that's why you see them both at the end, they're not ghosts, they're real, and never died, or killed any 1

and to answer the first part of your question?

no there's no significance

I thought it was clever too..love the creepyness of it...the music was good to.