Your list seems quite comprehensive. When Groucho was a baby, maybe 4-5 months he developed a case of the "squirts" so it was off to our vet. She did some tests, including FIP (negative!!!) and put him on Feline-ID. It's a prescription food both, wet and dry, that you can only get at a vet's. Groucho loved it and did very well on it. Maybe Angel just needs a chow change. Prayers being prayed that your vet can figure this out and get Angel to feel better very soon. Please let us know what happens?