I'm guessing a blood chemistry screen included SAP, SALT, glucose, creatinine, BUN, Ca, P, Na and K. You didn't mention FeLV/FIV tests.
IBD and intestinal lymphoma have similar clinical signs. Intestinal biopsy is the best option for diagnosis, but benefit may not outweigh the risks. Why did you stop the pred?Dose of pred may need to be increase or other immunosuppressant started along with maintenance metronidazole.
These are some thoughts in addition to what you and your vets already have on the list.
I wish you luck.