One thing that now comes to mind with Sunshine and Jenny is that there is a heart condition -- Nicki knows more -- that can affect cavaliers and cause a sudden 'blow out' of the heart valve, typically around age 3-4, with no advance warning. A dog that had just come into my rescue died of this about three years ago -- he and another came in from a breeder who was rehoming some dogs. He basically was in the vets at the time and his heart just failed -- very unusual for a young dog like this. Nicki and a breeder friend were aware of this issue in the past with some cavaliers. It isn't common and there's really nothing that can be done about it -- the valve just goes when they get around that age.
We did not do any tissue samples as my vet is small and I would have had to let the body go-this I could not do as I was not prepared to lose her so suddenly. Anyway, the necrospy was inconclusive and the vet suggested it was heart attack or stroke.