[anonymously]

Do you have a veterinary school near you? I would consult an Internal Emergency Medicine Specialist.
It does not sound like the regular vets have been very helpful, and if you have been going back and forth for over a year without positive results…….
Do you have a diagnosis?
That is the first thing I would want. People complain about the cost, but at least with a specialist or an emergency clinic, they do the testing and usually you are in and out of there in an hour or two with a diagnosis and treatment plan in place.
Her symptoms may have absolutely nothing to do with her diet.
PS: I might consider allergy testing by a board certified dermatologist, I’m not talking about mail-in saliva and hair tests.