I am sorry about the problems! Sounds so incredibly frustrating.
A couple of thoughts on FLUTD.
Approximately 70% of cats with FLUTD will positively respond to canned food versus dry food. The increased water intake can help reduce the concentration of the urine which can reduce the inflammation of the bladder. Addition of water to the wet food and use of a cat fountain can also be beneficial.
Also DHA (an omega 3 fatty acid found in fish oils) is thought to help reduce inflammation of the bladder wall. For this reason it is added into some prescription urinary cat foods.
Cosequin is occasionally used with anecdotal success. This comes from human medicine, where some women with non-bacterial bladder wall inflammation have shown to their bladder walls deficient in a glucosamine like connective tissue and respond to a supplement close to cosequin. For this reason, some cats are placed on cosequin. Cat specific testing has not been done to my knowledge.
If FLUTD is the cause, then some precription medications can help. Some that are used include amitryptilline, prozac and prednisolone.
I hope this helps.