We saw a new vet yesterday, who prescribed an appetite stimulant (ciproheptadine), and Royal Canin Recovery RS, which Xumi much prefers over the Hill's A/D. The vet was mostly concerned about the weight loss, and I tend to agree. She recommends a "wait and see" approach, and feels the Horner's will improve with time. She's only had 2 cases in her career, both dogs, which recovered spontaneously within days. She's at a loss to explain the deafness and neurological issues but agrees that it's related to the ear flush.
So, I have one other vet in town I'm interested in evaluating, and will try for an appointment next week. I'm not sure about the "wait and see" approach, but on the other hand, I'm not sure about subjecting Xumi to a lot of tests and procedures just to find out exactly where the nerve damage is, if there's nothing that can be done about it anyway.
Xumi is showing slow but steady improvement in her food intake, coordination, and general mood. However, she's still completely deaf, and has very low energy. Poor little girl.