OMG taxcat, one of my cats (12 1/2 yrs old) is going through pretty much the identical process as yours, except she hasn't vomited in the past week and hasn't been on prednisone, and both I and my vet have gone through the same thought process as yours: vomiting = hairballs or eating too fast; licking all the fur off her abdomen and inner part of her hind legs = behavioral; drinking more water = early kidney disease (through her creatinine level is a little high); more pee = drinking more water, not concentrating her urine; not eating = currently still a mystery, because she's been getting Periactin since last week.
Was/is your kitty very lethargic as well? It's the lack of energy that's worrying me the most. We've been to the vet twice in the last week - most recently just yesterday. Bloodwork last week showed nothing, xrays yesterday showed just a little constipation. She was given Pepcid ac and Periactin, and got subQ fluids on both visits. After going through all of the results, the vet mentioned that they could test for pancreatitis but didn't seem to think that could be it, so I didn't request a test. But there's been no improvement. I called the vet today but haven't heard back yet.
She does the exact opposite with food, though: she only eats watered down pate, so prescription canned foods will probably be ok. So your kitty is eating the duck and pea?? I tried that with my other kitty several years ago, when the vet thought she might have a food allergy, and she wouldn't touch it.
Dry food she just inhales without chewing, and that's what she always throws up too.
I don't think she'd eat raw food. She just won't chew. I don't know much about a raw food diet - why do you think it would be better for Isabelle?