I have a cat who was prone to UTI's once she got to be about 5. We went through everything you are doing- vets, tests, cleaning bowls, and on and on. Her issues were a major impact since I had to put a blue plastic tarp over every bed in the house. I found some cranberry extract at Pet Pals and that seemed to do the trick. I kept her on the supplements for a few months and then stopped. No more issues. I wan't going to pay more to the vet to find out why the problem stopped, but it seemed logical to me that she built up deposits in her urinary tract that continued to irritate her and the cranberry extract help it dissolve and finally flush out of her system. She's 12 now and we haven't had further problems. Well, okay, she's senile, but I doubt that was caused by UTIs 7 years ago.
With all of our running and all of our cunning, If we couldn't laugh, we would all go insane...
"Changes in Attitude, Changes in Latitude" Jimmy Buffet