A disability doesn't improve with medicine.

I think I understand what you're saying, but disabilities can absolutely improve with meds. Not all of them, and not all the time, but it sounds like that's what your ex-wife's experience was. She had disabling conditions, but it was more manageable when she took her meds. Despite this, there are aspects of her disability which were (are?) still challenging.

To me, THAT'S the experience which defines disability: you've got a condition which, despite yours and your doctor's best efforts, continues to express itself. And even then, I can see a lot of potential issues using that definition in terms of government support. It's compicated.