Lindie, do you mind my asking what your symptoms are? I have tested negative for antibodies, and my SFEMG was only mildly abnormal. My presentation is also "atypical" (my main symptom is weakness in my sides and back so it's hard to hold my upper body upright). My neuro lets me take the Mestinon, but doesn't really think I have MG.
Here's my experience: please take it with a grain of salt, because I don't quite know what it means. The doctor told me to go off the Mestinon for 12 hours before my SFEMG. Wanting to take no chances, I went off it for three days. Now I wish I'd gone off it for three weeks before, and here's why: after three days off the Mestinon, I didn't feel any worse, so I decided it wasn't helping and stayed off it for a month. During the first two weeks off it, I kept feeling muscle twitches (which I get when I take the drug). They got steadily more rare until they went away altogether. Then, after a month, I started the drug again and the twitches came back.
This makes me think that although Mestinon has a short half-life, and leaves your blood quickly, there is some after-effect that continues. If I have another SFEMG, I'm going off the stuff for three weeks before, just to make sure.
Then again, I'm not severe enough, nor does the drug help enough, that I worry about having a crisis without it. There are people on this list who shouldn't take a risk like that. I'm honestly not sure if the Mestinon helps me or not.
I sure hope you can get a diagnosis! I am in the same boat--my neuro doesn't mind keeping me on the Mestinon, but otherwise he's given up on finding out what's wrong with me, and he's the MG/neuromuscular disease specialist in my whole area.