Hi, cinusti! Yes, the Mestinon can affect the test results. You definitely need to be off it. I don't remember exactly how long, maybe someone else does, but perhaps a couple of days off, to be safe. Plus the day before your test, do everything you can to really exhaust yourself. You want to go in there as tired as possible. And anything you can do to tire yourself out while waiting to be called back for the test will help.
The ophthalmologist is definitely not ruling out MG. If he didn't think it was a possibility, he probably wouldn't have recommended a trial of Mestinon.
Keep us posted on the results of all this. Hope you get a diagnosis!
Hugs,