Hi Corinne,
The canned food may still be too processed a diet to be of benefit in a Damp animal. It would be nice if just for a week or two, she just used meat like from the grocery store, if the cat would eat it. If symptoms improved dramatically, you could then just create a balanced real food diet. And you'd know that Dampness was confirmed as the problem from a Chinese medical perspective, although with the diet change, herbs might not be required after that, but SRT sounds like it was the best tolerated and most comprehensively beneficial of the treatments, if something was still needed
One other idea, since this seems to be a Dampness case, is to look at Huo Xiang Zheng Qi San (Protect the Middle, if you're using Kan). It's the first choice for refractory GI signs in a young animal. It's still appropriate now If the Tritrichomonas is still present somehow because of truncated treatment. It also fits the worsening in January, when Wei Qi is at its weakest, allowing deeper pathogen penetration. It's quite aromatic, so the pill version from Kan may be the easiest to administer. It's a pretty strong anti-emetic and is used in China to combat morning sickness.
For the cat to have done well for two weeks on canned plus SRT, you have to be close to the solution. A resurgence of symptoms could suggest you're dealing with a leaky gut and secondary IBD to some degree, such that a sensitivity to the new foods was eventually acquired. Leaky guts imply probiotics and immune modulating formulas might help. You made an effort with XCHT, but one other consideration is Bu Zhong Yi Qi Tang. It bolsters Spleen Qi and slows peristalsis (to reduce soft stinky stools). It also fits mid-winter aggravation since it counters tendencies to Jue Yin excess and Wei Qi deficiency, both of which occur at that time of year. Since there is the remote chance it could aggravate vomiting, and since it is not that anti-Damp, I'd consider it a Plan D.
Plan C would be to explore Yi Guan Jian. If WLT and XCHT were truly aggravating, and the vomiting on WLT was not just worse because Cerenia had been stopped, then the tie that binds both of them is that they are drying. If a cat had Stomach Yin deficiency, they could be aggravated by those herbs. Stomach Yin deficiency can also worsen in January, and is accompanied by Heat that can produce inappetance, vomiting, and colitis. Since the formula is potentially Dampening, I would hold it in reserve in case those other ideas don't work.
In terms of dovetailing with what you've done and learned so far, the HXZQS is the place to start. Since it is also warming and drying, if the cat worsens on it, then YGJ might be the next consideration. If it doesn't help but doesn't make things worse, then given the wiry pulse, a moving formula like BZYQT might be the next best choice.
First and foremost, though, ask the owner to just feed meat for a week. If the symptoms don't improve, the cat isn't that Damp and you can zero in on YGJ and BZYQT. If the cat is much better, then continued SRT or a switch to HXZQS is more advisable.
Hope this helps!
Steve