I'd cut my losses, kat - you left (and well done for that), so I'd just pay up and move on (as I have done in the past....). I don't know the legal position, but since it was your decision to break, not his, it could be argued that it's you that initiated the fact that the wedding's not happening. I'd just get on with it.
Re the mortgage, if I were you I would make a schedule of everything you've been liable to pay out on since you split, which might make his half-a-month's mortgage claim look pretty pathetic. If he should sue you which he sounds rat enough to try it, then at least you'll be prepared.