Oh honey in prayers. A medical problem does make sense, and some of the ones you listed are a " quick fix." ( addisons and diabetes isnt..)
I agree with your vet, and I do not think its behavior drinking. Some of those tests it takes like 48 hours to get them back with the results. Did your vet mention cranberry capsules? It would like be one capsule for up to a 50 pound dog. While it helps the kidneys to work better it might help a little. Usually its more for moving crystals etc. I am not sure what he suspects the most, so before giving it to him, check with the vet.
I am so sorry- thank you for the pm, and I will both both of you in prayers!
** edit- it does sound like to me your vet is doing everything they can to find out what is wrong here.. I know its hard to wait- but the tests will hopefully show what is wrong..**