Oh Stacwase - my heart goes out to you - what a problem.

If everything else has failed - like it did for my daughter and her lovely furlady Tabitha - the vet put Tabitha on medication - can't remember what it was - buy hey - the problem finished!! This cat had peed everywhere - beds, carpets, sofa, chairs, towels, washing just taken out of the machine - an endless list. And then - nothing, she went back to using her litter box - they had to take the lounge carpet up and a new one in!!! As long as she's on the medication she's a sweetheart.
Maybe the vet would have the answer.

Lynne