I've had no breed related health issues so far. I feel very lucky, as she was a rescue. But, I knew if her heart was good at 5 years old, we were probably ok. The safest option is to go to a breeder who does all the health testing and follows the protocols, but that can be hard to find even among breeders who otherwise do everything right. I also have her on supplements to help try and prevent issues. Many Cavalier owners I know give 30mg CoQ10 daily. When they start depends on the dog, its origins, and the owners preference. If you do it, make sure to get the gel capsules, they are absorbed best by the body. The stats are 50% of Cavaliers will have a heart murmur by age 5. Its the early murmurs that are an issue in the breed, many small breeds get murmurs later in live. Kara is nearly 10, and so far murmur free, for which I am thankful. Life expectancy varies by the lines your dog comes from, and of course, any health issues. I've heard of Cavaliers in their late teens. But most I think early teens is a reasonable expectancy. Say, 11-13 or so.
As far as stubbornness and high chase instinct, that has NOT been my experience. They are descended from bird dogs, so can be a little "birdy" at times, but they don't have a high prey drive like a husky or terrier would. They are eager to please, but sometimes a bit slow. I know owners who do very well in obedience and agility with them. Some Cavalier breeders suggest they should never be trusted offleash, but I believe this is misguided. Bird dogs are bred to work offleash. Cavaliers are not much different. With proper training many Cavaliers do. Mine has a good recall, and enjoys off leash recreation.
RCR, I seem to recall your story. What that dog went through probably would have turned any dog unstable. Thats not standard for the breed at all. Cavaliers are known for being happy, outgoing, friendly, loving dogs. Have you looked into Cavalier rescue? They are very good about taking in just about any Cavalier, even those with big issues. And they would likely have an experienced foster home that could work through her issues without having to worry about harm to children or other pets. If you are interested, I could see if I have any contacts who could help.