Around a year ago, I found a starved cat (2 years old/neutered) at a campground and took her home. Almost immediately, I noticed large urine spots wherever she sleeps. I took her to my main vet who checked her urine (they had to sedate her) and while sedated, she leaked urine all over the table which they said is not usual and started her on incontinence meds (Phenylpropanolamine). I took her to a second vet who ran more tests, again found nothing.
From there, I took her to a specialist who did an ultrasound on her and ran some tests. $2k later, they were able to find nothing. They told me that my only other option was to go to a state specialist vet but that would be a $3k test and if it was the issue a $7k surgery which COULD help but wasn't a positive. I was warned that they rarely do this type of surgery on cats since they rarely see young cats with incontinence. I've tried diapers, but it's hard with a VERY active cat who DOES use the litter box normally when she's awake.
But the problem comes in that my older cat believes that this cat is marking everywhere and it makes her so irritated that she has started peeing and marking everywhere in response. I have to keep her on anxiety meds or she will spray all of the walls and basically pee everywhere my younger cat has leaked. She will also fight the younger cat which resulted in my dog attacking them to break them up (which was extremely stressful and I've kept them away from the dog since).
I've tried finding another home for her but no one wants to take an incontinent cat. I live on a huge property with a horse barn and thought I could make her an indoor/outdoor cat (I know keeping a cat outside is not ideal but I need to do something) so she's not leaking in the house AS much but she HATES the outside. I built her a catio to encourage just being out of the house but she hates that as well. Does anyone have an experience with any of these issues and have any ideas? I hate keeping my older cat on anxiety meds, the house is starting to smell like urine, the younger cat is SO sweet and fun but I need to figure out how to do something. I don't think keeping them separate at all times will fix things because the older cat can still smell her urine (and I would never rehome the older cat, she is my world). Thank you!
(Adult, USA, I can afford vet care to an extent (would possibly be willing to spend the $10k if I was positive it would fix it but they weren't positive)
If it matters, she eats Purina HA. I have a cat that has allergies and when I got this one she has a lot of GI issues so I had her start the HA too and her GI issues went away so I've kept her on it.)