It is of course concerning that your GP has "limited experience" with MG, but then 99% of GPs are in the same position. I hope that he is liaising with an expert, with more knowledge.

For a GP to give a dx of "seronegative MG", would be unusual, that sort of decision should really be left to an expert Neurologist.

Your understanding re Mestinon is correct, but some people can and do get along fine for many years on just Mestinon.
And, as of this moment, there is NO treatment for Mestinon, that cures it. All teatments simply address the symptoms.

Simply put:
Mestinon, gives the ACH longer to work.
Immunosuppressants, suppress the anti-bodies that attack the receptors (but suppress your immune system in general)
IVg simply swaps out the blood or plasma with the bad antibodies, for blood or plasma without them.