I have also had "midly" abnormal test results --modulating Abs of 24, with 20 being the upper limits of normal, and a SFEMG in which a few fibers were abnormal. My local doc says he is not convinced it is MG, despite the fact I take Mestinon (which helps alot) and get IVIG every 3 weeks.
Fortunately I'm going to get another opinion (although at this point I'm not holding my breath).
Have they at least tried you on Mestinon? With your other autoimmune diseases, are you on prednisone or other immunosuppressant?
I know it really sucks being told that they don't know what is wrong...and consequently do nothing to help.
Hang in there