I've had several battles with Cystitis this year. It doesn't sound like you are going through the same things as my Tilly does though.
Tilly's bladder shrunk so much she couldn't produce enough to soak anything.
Just small blobs of heavily blood stained drops. She mostly took to doing it on Terry's discarded shirts.
It broke my heart to see her straining and straining to produce a little drop of bright red urine. Sometimes that's all she did for hours... just wander to a new area, squat and strain over and over again.
Cystaid combined with a long acting antibiotic was clearing it up for short periods but it kept coming back after around a month.
Uncle Simon thought she may be carrying a peculiar virus not covered by the antibiotic injection and he needed a urine sample free from blood cells when she was inbetween episodes. Tilly wouldn't obliged by using the special litter so she had to be sedated and they took some urine from her bladder with a needle. It turned out she didn't have the virus, even if she did in the first place, so in theory Cystaid should treat it if given for long enough.
Uncle Simon said she needed to be kept on the Cystaid for a bit longer after she seemed back to normal. I sprinkled in on her food at the first sign of trouble and she was back to normal in a few days. So I kept giving it to her for a few more days. Touch wood she has been free of Cystitis for around 3 months now.
Jumpy is a swine and nibbles it too but Simon said it wouldn't do him any harm.
So long as Tilly got most of the tablet's worth, that's good enough.
Big hugs to Willow. Cystitis is horrid.
Fingers crossed they will find whatever is causing it soon. That Cystaid is wonderful stuff... can you not confine her to one room while she eats? Even if she gets just some of the powder inside her it's going to help.