I'm glad the Mestionon is helping you. That is probably one of the best ways to know if you have MG...to see if it helps you.
I brought up being overweight only because some of the specific symptoms you listed have become problems for me since I gained a lot of weight; specifically sleep apnea, back pain, the breathing issues and increased heart rate upon exercise. They weren't issues before and they got better when I lost about half of what I've gained.
I just know personally, I'd much rather have weight related issues than an incurable autoimmune disease any day! So much is completely out of our control; to me anything that can be helped is a plus.