Jed, I couldn't work out how to "multi-quote" your post, but thank you for it!
For information, I know a lot of Cavaliers in SA who have lived long, healthy lives into their teens (13-15 years), rarely needing to visit the vet. And when I say "a lot", I do mean "a lot". I've been involved with this breed for 30 years (pet owner, then exhibitor/breeder/agiliteer), and attend the Cavalier Club's fun days where up to 150 pets and their owners may be in attendance. They come from all over the state, and from all breeders. Seeing all of these dogs, talking to their owners, gives a very good overview of the breed.
The SM issue is a media beat-up, and while none of us wants to see any of our dogs suffer, in 22 years of exhibiting and attending pet events, I would have seen maybe a handful of obviously affected dogs. I'll leave any further comment on the condition to anyone else with more experience of it. To respond to another poster, it can be tested for, but not in all states of Australia, and at up to $1,000 per dog.
Responsible, reputable Cavalier breeders are health testing as much as we can.