Sorry you are going through this. Your symptoms sound just like mine and more or less what is typical with MG. I am seronegative too. I was diagnosed by a single fiber EMG. I have a couple recommendations for you:
1. Go see a neuromuscular specialist at a large teaching hospital that specializes in MG or diseases of the neuromuscular junction. MG is very rare and most neuros have never seen it. Going to a teaching hospital where they see it all the time is very important especially if you are not the typical presentation like me in being sero-negative.
2. Statistics show that only 85% of MGer EVER show positive antibodies. And new MG patients may be seronegative at first and then turn positive later. You don't want to have to wait months or years for treatment waiting for the antibodies to be positive.
3. There are several different antibodies to the neuromuscular junction. ACH antibodies is the most prevalant-the commercial blood test done. Musk is probably the next highest. But there are others that have been identified in research that they do not even have commercial tests for.
4. The single fiber EMG is the definitvie diagnostic test for MG no matter which antibody you have. This test can only be performed and interpreted by very experienced neurologist that you will find at a large teaching hospital.

Your Mestinon dose is pretty standard. I take 60 mg 4 times a day and 180mg slow release at night. I am also being weaned off high dose pregnisone. Mestinon only temporarily relieves symptoms. It has a short effective duration, it does not build up in your symptoms and you will not see any cumulative long term build up effect from it. Everyones dose is different, some MGers take smaller dose more often, some are on a much higher dose than me. Here is what I experience from mestinon. It take about one half hour to feel any effect, then I notice how much clearer my vision is, it is easier to breathe and my muscle have more stamina. It is in full effect from 1 hr after dose to 3-3.5 hours after dose. Then I feel a very drastic weakening for the last .5 to 1 hr before I can take my next dose. Mestinon only temporarily relieves symptoms, it does not get to the root of the problem-antibodies. Most MGers use mestinon and another type of treatment to suppress antibodies such as pregnisone, cellcept, IVIG, plasmapharesis, etc.. It takes a long time to find the right combination that works because each person's MG is so different. I have been on meds for over 6 month and my MG is still not under control. I am trying IVIG shortly.

I hope I have not given you too much info too soon. But MG is a life altering diagnosis and I always felt better with more info so I can make the right decision. Let me know if I can be of any more help.

Good Luck,
kathie