I completely understand the frustration. I won't bore you with what I went through during my crisis but it has similar undertones. Since you didn't have positive antibodies, they want to make sure you have MG before they do plasmapheresis. I know, do they want you to die proving it?

But that's their world of little algorithms. Mestinon and steroids are fine for mild or borderline MG cases. But no antibodies or positive EMG's and they go right for the risk managers (the ones who make sure they don't make any mistakes which might leave them open for lawsuits).
What about your TSH? Have they tested that again? You have antibodies, so any thyroid changes can make you worse.
The problem with doing an EMG now, after having steroids, is that the job of steroids is to make the EMG's and antibodies go negative. Some patients are so bad off that they will have a positive EMG but a regular EMG is not as sensitive as a SFEMG. Have you had a SFEMG? Just let them do it but make sure they do it in a very weak muscle. They won't do chest wall muscles because it's too dangerous.
Careful with those higher doses of Mestinon. To jump from 60 to 120, for example, can cause a cholinergic crisis! Sometimes it's not the dose amount but keeping a consistent frequent dose of it, like the every 4 hours. Talk to your neurologist about this before you do anything too drastic. And while I know some MG patients can handle doses above 100, there are a lot of studies that show anything over 100 can flood the neuromuscular junction too much, causing that cholinergic crisis (too much acetylcholine).
Try not to take what they're doing personally. I know, not easy. They'll "have to" do plasmapheresis and ICU if you get worse. You're on the edge of that right now. Just rest, take it very easy, don't stress out, keep cool and give your MG a chance to get better. I hope they will figure out something to help you get better. Hang in there!
Annie