Recurrent bladder infection.

I have a four month old pug puppy that has had symptoms of UTI since 4/8/13. She is still having symptoms and I tested the pH of her urine at home and it is off the charts--over 8. I have a vet appointment scheduled for this weekend but I am not hopeful that they will do anything more than give her antibiotics. I have been researching like crazy since April of what things to ask them every time I bring her to the vet and now I'm running out of ideas. Here is the history:
4/8 (Monday) Noticed increased urinary frequency, monitored her until I saw pink urine. Brought her to after-hours vet, no fever, came home with 1 week of Clavamox liquid, no tests done.
Submitted urine sample to regular vet the next day, noted white blood cells, no crystals. Puppy responded to Clavamox then deteriorated again the following Saturday before the dosage was complete. Brought her to the emergency vet again for pink urine and passing some blood clots, again no fever, received another week of Clavamox. They did a quick bladder ultrasound and thought they saw a stone. Kidney blood panel showed no unusual values. I made an appointment with internist for the following Tuesday even though she had a scheduled vet appointment on Monday. On monday, her regular vet x-rayed her--no stones. Knowing some stones don't show up on x-ray and fearing liver shunt, I took her to the internist on Tuesday for full abdominal ultrasound and full blood panel. Ultrasound showed 0 stones, for abnormalities only one small polyp on one of her kidneys which the internist said looked typical. Blood panel came back normal again with very slight anemia which the internist said is common in a young puppy. Continued with the second 1-week dose of Clavamox, puppy responded well, was fine for >1 week after the Clavamox was finished and then began presenting symptoms again. Took a sample for urine culture with specificity--the sample was FULL of very visible sediment. Nothing grew in the culture. Was given a three-week dose of Clavamox again. Again she responded and symptoms subsided. The three weeks is almost over, and she is starting to be frequent again (though her urine is a nice color) I tested her urine myself a couple nights ago and found pH > 8. I have now added d-mannose and a pinch of vit-C powder to her food, switched her food to canned (I wrote to the manufacturers of her first food to find out dry-matter ash content, and found it was 9.7), and began adding organic ACV to her water in an attempt to bring her pH down on my own. Too soon to tell if there will be any results from that, I think.

In my latest visits, the vet(s, since now three vets are involved) started to talk about the possibility that she may need to go on a urinary diet, but they didn't want to because she is still so young. One of the vets basically said that they would continue to throw antibiotics at the problem (they said they have not switched antibiotics because Baytril can impact bone growth), when I voiced my concern that nothing grew in the culture, she basically told me she didn't know what to think.

What else can I do? What can I ask her vet? After all my research I am left with the possibility of some superbug bacteria or bladder cancer. For every suggestion there seems to be a reason why it is unlikely--why go on a urinary diet if no crystals in her urine (meaning not struvite)? Why give her d-mannose if e. coli did not grow in the culture? Why biopsy her bladder lining for cancer cells if she is responding to antibiotics? Of further note, we also spent the first three weeks at home with her 24/7. She is pee-pad trained so I have been lax on outdoor potty training because I don't want her to feel like she has to hold her urine and make this worse. She is penned in our kitchen with an accessible water dish and pee-pad and everything harmful out of reach when we are not in the house, which is a maximum of 4 hours at a time. She sleeps in our bed and has had only one accident in the bed EVER and sleeps through the night for 5 hours. To get her more interested in drinking I have trained her to drink from a spray bottle while we play.

Thank you for any ideas you may have...I just need to figure this out, no matter how bad the news is!