I'm so glad the vet has kept her overnight on fluids and will continue testing to see what is going on with your cat. Sounds like you are trying alot of different ideas to get to the bottom of this mystery.
KittyKatharine's idea of Famotadine (Pepcid) is a good idea and worth a try. It might be that her tummy is just too uncomfortable for her to stomach food.

I was also wondering if she might be dealing with Irritable Bowel Disease. I have a cat with this and when he was first found by someone (before I adopted him) wandering the streets, he was about 5 pounds and had horrible diarrhea. He wouldn't eat etc.. They put him on a steroid called Metronidazole and on a diet made for cats with IBD and within time he got better and better.

I'm not sure if there is a test to find out if your cat has this as in my cat's case it was a diagnosis of exclusion. His disease also seems to be emotionally connected. If new things happen in his life, he tends to relapse. (We recently lost a cat to Rainbow Bridge and adopted two new cats.) It sounds like there have been some changes in your world with marriage and other things.

Please keep us posted. Prayers for you and your kitty.