>>My guess--he'd gotten rid of the old one the night before, because it was drenched in her blood.<<
If this were true, he would have been arrested over a week ago. I doubt, given all the other stuff, he would have done that great a job keeping blood from every other place there.
As of right now, I honestly don't think the police have a clue what happened. That may or may not change, but until the confirm she is actually dead, this case is going nowhere.