My elderly cat was on cerenia for nausea - it helped a lot.
I was fortunate enough to work from home and her dementia didn't progress to the point where she couldn't find me. She did have periods of late night activity, and the vet told me to give her melatonin. It worked until it didn't, and it's hard to find a brand with tiny enough pills for a cat to swallow, but might be worth a shot. It sounds like your other options are earplugs or letting her pass on. Only you can make the call when it's time. I sincerely wish you the best in this difficult time.