I can not think of a single thing that long term peeing that doesnt turn into blood etc to indicate a different problem. Petetuatary, thyroid, diabetes.. those yes- but the last one has been ruled out. Dogs that are hypothyroid the side effect of UTI shows up alot- but I am at a loss here.
Your vet looks like they have the gamma covered.. I know the waiting is hard, but at least you will know and how to treat it.