Kathie, Please take it easy! Sleep is like a prescription for MG; without it you get worse.
I hope the test goes well. Don't worry about it. It'll be what it is. Even a negative SFEMG doesn't mean you don't have MG. You obviously had a highly positive response to Mestinon.
If you get so bad that you can't breathe well, are very weak, etc., well, you know what you should do! Don't risk your health for the sake of a test. Call your neuro if you need to too.
Let us know how things went. Whatever you do, don't wear yourself out before the test by walking up multiple flights of stairs or exercising or whatever. I think going off of the drug will be quite enough!
Annie