Welcome to the forum, I am sorry to hear about your problems with your cat. Hopefully you'll be able to find some answers here that will work. All I can do is suggest something that worked for me about 10 years ago with one of my cats. As a kitten, Missy's system had been overwhelmed by stress, vaccines, spaying, and over prescribed antibiotics. This started her liver to shut down, and when the blood work was done the liver function test was right through the roof. The vet I had at the time gave me 2 options for treatment: the first was the normal allopathic medicine using pharmaceutical drugs, and the 2nd was using alternatives. I asked her for her opinion, and she said that she hadn't had much success with pharmaceuticals in this type of situation. She went on to say she wanted to consult with some of her colleagues stateside, but she thought doing homeopathics would work.
The long and the short of it. We went to a homemade diet, added a few supplements, and the main ingredient was Milk Thistle to my way of thinking. The whole idea behind it was to eliminate the stress on the liver caused by store-bought foods and previous medications. The whole process took considerable time. You could see a slight change after a week and a half but it took about the best part of 3 months before she was back close to normal. The milk Thistle I used at the time was called Bio - Foie. It was made in Québec, and with that brand I was giving 4 to 5 drops in about 1 1/2 mls of distilled water 3 times a day to start. I have since switched to another milk Thistle made by A.Vogel. This one I use 2 sometimes 3 drops in about the same amount of water and it works quite well also.
I can't advise you on using milk Thistle. But if you're coming up against the wall it might pay to find a vet with some knowledge in alternatives that you feel comfortable talking to, and run it by them for their opinion. It might be something that would work for you as well. It's worth thinking about.
Please keep us informed we would like to know how you're doing, and hopefully some other members will add their ideas as well .
Animals are such agreeable Friends.
They ask no Questions. They pass no Criticisms.