You may want to talk to Mycavyangels she has a few cavaliers. She has cats too. A co-worker of mine had one, the sweetest little pup, she died suddenly before she was a year old. I believe it was the heart-valve issue that you mentioned so definitely look more into that.

I'm a groomer, they aren't an extremely common breed around here but I've met quite a few. I've never met a mean one, every one I have groomed has been very good natured. They seem to be really sweet dogs. I've noticed there's a couple of different types, ones that are bigger with a stocky build and ones that are small with a thin build. I'm not sure if there's anything to that, it just seems like the size varies quite a bit.