What ended up working in the end? We finally got into Mayo, and they suggested an SSRI (at a fairly high dose). She also figured out that she has a few food triggers (yogurt and freshly baked bread are 2 bad ones). She also discovered she has double vision, and now wears prism glasses. We think the combination of double vision and whatever brain chemistry imbalance she had was "overloading" her vision and vestibular systems, and she would basically just shut down. Treating both of those seems to have (mostly) alleviated things for her.
I completely understand what you've gone though and how frustrating it is, especially given the very vague criteria for diagnosing and treating vestibular migraine.
The way one ENT described it, I have 40% degradation on one side. If it’s constant, your brain adjusts and you are essentially ok. If something happens to vary that then your brain can’t adjust. My theory is that you add inflammation or migraines or epilepsy or all of it and the brain can’t compensate.