I'm sorry your cat & you are dealing with this
Is there a trained animal physiotherapist that can assess your cat? Nerve issues can sometimes be a result of the spine misaligned or a very tight muscle pinching a nerve etc. Someone trained in animal chiropractics, intramuscular stimulation, acupuncture, or proper animal massage therapies might be able to offer some relief.
Did the vet find any inflammation in the bladder or urinary tract? Any crystals or stones in the bladder or urine? Did the blood/urine tests show signs of kidney insufficiency?
If they are still unsure of the cause, are they willing to call in another vet for a 2nd opinion or you can always opt to take her to someone else yourself.
Ask your vet if they offer/will accept a payment plan, that way you can manage the vet bills better especially if a trip to the specialist is in order.
for the biopsy results