I do not have rabbits with kidney problems, but I have failing kidneys. Ive likely been through many talks with my doctor that you are having with your bunnys vet. Im sorry the both of you are going through this
This may come off rude, and I really dont mean it to, but with the risk of cancer and her kidney not getting worse, why not opt for the biopsy instead of treating for UTI? A biopsy will be able to tell you much more than whether a UTI test works or not. Often, a biopsy can tell you whats causing the problem, which could be anything from cancer, failure, disease, to a simple stone or need for diet change (etc). Having the biopsy done is better to get done sooner than later to avoid infection/cancer spreading, and to treat the issue asap.
Regardless of what order of process you choose, there is a high chance she will be ok. Like humans, rabbits can live perfectly normal lives with only one kidney if it came down to her needing surgery 