I'm sorry you are having this problem. I wish I new something that would help. I went through something similar with a cat we fostered for a few weeks. The rescue let us foster her as a trial basis for adoption. It didn't work out for several reason but mainly because she decided bedding was to be marked with her pee. She first peed in the middle of a papasan chair that Ripley used to sleep in all the time, then she started peeing on Disney's blankets in her dog bed and then she peed in the middle of our bed. I took her to the rescue's clinic to be tested for a UTI but it was negative. I tried keeping her in the guest room for a few days to "retrain" her on the litterbox and it went well but as soon as I let her back out she headed straight for Disney's bed and peed it in. She was also very dominant with Ripley and Disney (my only other pets at the time). She scratched poor Disney on the nose.

After talking to the rescue group we decided she may need to be an only cat so I returned her. She was adopted a few weeks later and it worked out well.

I'm by no means saying you should rehome your kitties, I'm just saying I can understand your frustration. I hope you can get it resolved soon.