I really do feel for you, it is a horrible situation to be in and one where there simply isn't a 'right' answer.
I had the experience a few months ago where 4 vets including a very pricey specialist failed to diagnose his problem, but I finally went to a friend's vet who more or less instantly knew what the issue was, just from reading the case notes!
I am left feeling incredibly grateful to the last vet, and wondering why on earth the other 4 vets including the specialist, who all had the same information, failed to realise what was wrong.
What I am trying to say is that it is entirely possible for vets, even specialists, not to come up with answers. If you decide to visit the specialist as you say it might still be a shot in the dark and is likely as you know, to cost a fortune and require further tests. On the other hand, you need to be comfortable in making a desicion and possibly living with "if only." I am repeating myself and saying there is no right answer.
However, Rasha is only 9, otherwise apparently fit and healthy, all the tests so far appear not to suggest cancer and heart disease is usually something which can be stabilised on medication, if not cured outright through surgery. I'm certainly no expert but it does sound as if this could possibly give you a positive outcome so that at least you are not throwing money away and putting her through more tests for nothing.
I will continue to keep you and Rasha in my thoughts and prayers.