Good, sounds like the doctor is being thorough.
What other disease state does Mestinon help? Oh, let's see . . . Mestinon is sometimes used to increase peristalsis. The drug can increase blood pressure in POTS. In some people, Mestinon temporarily (for maybe days or weeks) may help forms of ALS.
HOWEVER, Mestinon helps ONLY MG, LEMS, and some of the CMSs. Period. Of course, there's always the placebo effect.
I know how tiring and frustrating it is to not have answers. Now you have to wait a couple more months. BUT this guy sounds like he wants to figure this out for you. And that, "I don't think it's MG" thing has been said by so many doctors whose patients end up having MG.
This doc is lending you a hand OUT of the rabbit hole. Just trust what he is doing.
In the meantime, take it easy and enjoy life!

Annie