sorry about your cat. i don't have any short term advice (looks like you got some good advice so far) (the d-mannose sounds good but i'd google and make sure it is ok for cats)

but, longterm, my cats like this cat kibble called "wysong uretic". it's a kibble that supposedly helps with urinary problems.

i don't rely on kibble, but i leave a little out for snacks. the barf diet seems to be the best diet for dogs and cats if you can do it. my cats are about half transitioned to it. (bones and raw food). cats are so picky that it is really hard to change them! haha good luck.