There are so many questions hanging over Snoopy already regarding her present and future health that I would go for the echocardiogram and get some answers - bad or good. I'd just feel on firmer ground that way. You may get good news so that you can plan Snoopy's spay, or a few months of pill and then spay. Or you may find out that she has more problems than are apparent and that will help you and your vet plan everything in her best interests. She will need to be sedated for the test, but they should be able to do a very light gas anesthesia or sedative, as I am sure your vet has assured you.

If you do decide on the echocardiogram, I think everyone on PT will be holding their breath on the day she has it done.