Wow, I can't imagine the pain of what you've been through. I'm so sorry.
Yes, stress like that can bring on a disease.
What kind of doctor threw Synthroid at you? The same one that threw the Acid blockers at you? The best doctor to evaluate endocrine issues like hypothyroidism is an endocrinologist. And gastroenterologists should thoroughly evaluate any GI tract issues.
What if you've had Celiac Disease this whole time? There are so many reasons for digestive problems.
I would highly suggest you get your B12 checked and possibly other vitamins like folic acid, vitamin D and iron, albumin and calcium/electrolytes. You've been on those acid blockers way too long. They aren't meant to be taken on a long-term basis!
This is the interesting thing about diseases. Simply because symptoms stop, that doesn't mean the disease state or underlying cause has. So stopping meds that would help you and are meant to make symptoms lessen or go away is not exactly the thing to do (like the B12).
It might be time to get a 2nd opinion from a different internist.
You might have had MG a long time. It's hard to say. People are really good at dismissing symptoms or just putting up with them. I hope you will get the best care possible for everything going on. Whatever you do, don't stop the acid blockers abruptly! You need to be followed by a very good doctor if you plan on doing that.
When I had an upset stomach from not enough acid, before I knew that was the problem, a doctor told me I had gastritis and should take Tums.

Wrong. Six months later, I started the Betaine HCL (a mild acid usually made from sugar beets) and everything got better from there and quickly.
You've been through a lot! Take good care of yourself. Again, I'm so sorry about your daughter and grandson.

Annie