It's so frustrating when you can't get any answers.
I remember with Evan, before they finally diagnosed his inflammatory bowel disease, we went through test after expensive test and all the results were inconclusive. We finally got some answers at a veterinary teaching college about 2 hours from where we lived. They ran a few tests, too, but they see so many difficult cases that they could narrow things down very quickly and knew which tests would best lead to a diagnosis. You wouldn't happen to be near a veterinary teaching college would you? Might be worth a try if you are.