Touch wood, Mimi's new regime seems to be working. She won't eat any of the easily-available GI foods and in fact immediately vomits if tricked into doing so. But she's on single-source-of-protein grain-free high-meat-content stuff - Untamed, Katovit, Animonda, Smilla - and I'm seeing more empty dishes and fewer apocalyptic litter boxes. Her appetite's increased and she's more lively and obnoxious. (It scares the hell out of me when a spicy animal suddenly becomes placid. Never a good sign.)
She gets a treat food like Applaws with probiotics in the morning to get her going, then complete dry food for lunch, complete wet food for tea, and a Lick-e-Lix with her steroid in. (The Lick-e-Lix isn't ideal but we need a method of getting pills down her that doesn't involve my nervous and inexperienced partner wrestling the pointy end when I'm away for two weeks.) She's currently on the maximum daily dose of Prednisone and it really seems to be helping. I'd like to get her onto a lower maintenance dose ideally but that's a future problem. I mean we now have an immunocompromised cat, but we have a living and eating cat. Who lives indoors and never meets any other cats to get diseases from.
Current working assumption is kitty IBD/chronic enteropathy. Lymphoma remains a possibility but the vet couldn't feel any masses; she's almost impossible to examine (gabapentin makes her wobbly, but no less violently angry, so I put on a lot of layers and give her a bear hug while the vet works as fast as possible and she chews my glove); it doesn't always show on ultrasound/biopsy; and treatment would be the same anyway (steroids + try to find food she can actually digest).
Now we just need to get some weight on her. I'll be a happy woman if she's put on a couple hundred grams before her next check-up, but long-term goal is to regain a kilo.