I am glad she is doing a little better! I just posted in OT, one of my cats has exocrine pancreatic insufficiency (along with CRF). We (vet & I) have been tinkering with this for a couple of months, and have finally found a combination of digestive enzymes and food (Purina EN & Fancy Feast ) that are working for him for now. I opted not to do the test, just try the treatment.
Maybe some digestive enzymes would help Happy keep some weight on. As I understand it, the pancreas stops producing the enzymes that help break the food down, so it passes through the system with out being absorbed. (sorry if you already know all this!) With Happy already not eating much, not a good scenario.
It sounds like there is some food sensitivity there, so maybe some easily digestible foods with some digestive enzymes?
for something to work!