With mental illness there is no way to ever know if the diagnosis is correct. The reason for this is that many physiological conditions can cause the same problems and doctors rarely look hard enough for them. At some point you just accept that's what it is and treat it as a mental illness, but you never can be 100% certain it wasn't, say, a thyroid problem or a blood sugar problem or a hidden virus. Now, about migraines. They are a vascular disorder, not like other headaches. This is a simplification, but think of them as little strokes. They aren't that severe, but are caused, as strokes are, by problems with blood flow in smaller blood vessels. What happens with a migraine is, for a variety of reasons -- it could be mental, it could be a food allergy, it could be other things -- the blood vessels constrict. When the stress eases, the blood vessels begin to open up again, but they open up first near the head and later in the extremities. This causes the vision problems that precede a migraine and the awful headache. That's why one of the more common treatments is biofeedback to teach a person to warm the hands so the blood vessels in the extremities open up, preventing the blood from gathering in the head region. The cure for me was a doctor many years ago telling me to learn TM, a form of meditation, and it pretty much got rid of them. It's why exercise also helps. Basically you try to even out your blood flow. In your husband's case, this has been going on for a long time, and he's also been on a lot of meds, all of which have side effects. He might also have had a stroke or mini-strokes. There's just a whole lot of things it can be. So while he may have developed the symptoms associated with conversion disorder, they are also symptoms of migraines and some of the treatments for them. Nobody here is a medical expert, I don't think anyone on here can tell you how to get better. You need better expert help than us. Conversion disorder, as with most mental illnesses, has no known cure other than therapy that changes a thinking process that might be creating the stress that manifests itself in this way. Medication can help with symptoms, but none of them are cures for mental illness. Botox has its own side effects, some of which also match some of the symptoms you're describing. So no, you can't be certain, and there is no certain cure, but keep trying, you often find someone who can help if you try hard enough. It would be good to know what caused those initial migraines, though, because hearing loss doesn't jive as far as I know. Best of luck.
I want to convey how much your post moved me. I mean, really. You and your husband have been through so much. And I can sense how he is trying so desperately to get better. It sounds like you have had expert care and have been thorough much testing and work to find exactly how to help your husband.
I have one thought. We have a family member who because of anxiety had a complete shut down of their autonomic system. Vision was greatly impacted. This has something to do with blood vessels in the eyes. Here's an old medhelp post on the subject: https://www.medhelp.org/posts/Dysautonomia-Autonomic-Dysfunction/visual-issues/show/589552 Here's
a really simplistic information page on it. https://www.healthline.com/health/autonomic-dysfunction. Likely not related but food for thought.
From my reading about conversion disorder is that he 'should' recover given time. He's not doing anymore CBT? Was he released from therapy? And are doctors aware he no longer takes some of the prescribed medication. I think that is great but everyone needs to be on the same page.
How many years has this been going on?
My thought is if Western medicine finds nothing wrong with him, them Eastern medecine might help.
The East perfected treating diorders as best they could before the advent of western medecine. It might be a brain thing, which Eastern medicine has a lot of experience in.
Also possibly the 2 books by Norman Doidge, M.D., on how the brain heals itself, may be useful.
Meditation, or meditative movement activities such as qi-gong, tai-chi, certain types of yoga, seem to have some positive brain benefits.
(My wife often had migranes. We never figured out what caused them.)