I hear you. My heart is totally with you. I loved my cat so much.
Yes, that was the final guess by the vet. My vet is amazing. He is one of the best vets in the entire area, and is very well known. I trust him completely. He's a very busy man, but took the time to write me a personal note after she died saying that it really upset him that he isn't able to cure these kinds of things.
Yes, we were also given the expensive option.
You sound like me, watching the cat very very closely for any behavior change. If the cat would take one sip of water, it would make my day! I was incredibly hopeful with anything positive.
Hang in there. I hope this works out for you and kitty.