We went through the same battery of tests when Evan was diagnosed with IBD, Aimster. The only difference was that they did the punch biopsy and the test for the Addisons on the same visit. So it sounds like your vet is following a logical progression to determine what the problem is. As in any diagnosis, until the final answer is found, the vets really are
making their best guess, then following up to rule it in or out.
I do hope the next test gives you something useful to go on.
I know how difficult it is to have seriously ill pet and not know what's wrong or how to treat it.