I think you need to sit down with some very specific questions that you can then ask your vet, write them down and perhaps ask for a specific plan in the light of the negative tests. I am not sure what they plan to do when she is sick again? How does that change anything?
You have done a lot of tests but maybe you just need a specialist to help work it out and get your moneys worth by getting some specific actions and ask lots of questions.
I once had been unwell off and on for some months. I spent a lot of time with doctors and loads of tests and scopes, all negative, trying to sort out what was going on. Eventually the doctors gave up and sent me to a specialist diagnostic physician and he had it nailed at my first appointment with him. He was simply amazing. What he diagnosed wasnt even on the doctors radar.