So glad that you are getting the second opinion. If you can, ask about using slippery elm, marshmallow root and red raspberry leaf which can be soothing to the sensitive membranes and help prevent apprehension at voiding/defecation
I second trying fluoxetine. My senior rescue cat had some anxiety issues that showed up in a lack of appetite, some odd habits with plastic and his stuffed toy frog, and peeing outside the litter box. He has been on fluoxetine now for over a year and it totally changed his life for the good. He doesn't hide when people come over, eats, still has a few odd habits but not near as consuming, and almost never pees outside the litter box.
So we got the second opinion, went over all the history, and the doctor did her exam. Ultimately she agreed with his history/age and her findings, pts wouldn't be wrong. So that is what my sister did. The vet heard something going on in his chest, and of course at my old job/vet I don't think we ever did chest xrays, only abdominal. So we were likely missing something this whole time. I never 100% trusted my old job so it would make sense.
I am very sorry for you and your sister. There were probably deeper issues at play, as the vet found. If you would like to post a tribute or remembrance to this cat in our Crossing the Bridge forum, you are welcome to do so. The thread will be locked out of respect for the cat.