I hope Dr Miller can see you soon... and I hope she has some positive news for you on management of your boy's condition. I suspect that there is no "innocent" cause for a murmur as serious as your vet heard but miracles do happen and Bobo was living proof of how they can manage with serious heart defects for longer that anyone could imagine. Donna Lawver (Summerwood Savannahs) had an F1 with HCM that lived for almost as long with pretty serious HCM (Hypertrophic Cardiomyopathy, the most common cat heart disease) with careful medication etc. These Savannahs I think are strong kitties that fight their conditions... with our help 
Paws crossed over here for a good outcome!
Paws crossed over here for a good outcome!