Hi Jaq!
I'm sorry to hear things are so stressful for poor Trixie! The poor gal has gone through so much these past few weeks.
My experiences have been different then yours, so I can only tell you what has worked for me. Mostly, I just want you to know that you are not in this alone.
I STRONGLY believe in the piroxicam and think it is what has kept my Annie around for so long. Just recently she has started to breath heavily and none of her vets had any idea why. When I read here on the board that it caused some dogs to pant, I was so surprised. Prior to that the only reaction I had heard of was upset--sometimes very--stomachs.
At any rate, perhaps there is too much change too fast for poor Trixie at this age. My opinion is that the carbs are less dangerous than the piroxicam is good. I hate to see her diet changed around again (and it is also so important that she eat--anything is better than not eating) But suppose you go back to her regular dog food (perhaps with the chicken breast added a bit) and try introducing the piroxicam again.
Because I have two dogs, I have always hand fed Annie her piroxicam and to make sure she ate it promptly, i would melt a cheese stick in the microwave and make it soft and warm and then wrap the capsule in that. Annie thinks it's a treat and the other dog gets a bit of just cheese as well. They think it's dessert!! (Oh that's the other thing, I feed the piroxicam at night after dinner--Again it's what has worked for us)
Good luck; I hope others post soon with their thoughts.
Marty