What exactly doesn't fit?

To me you sound like MuSK MG. Were you tested for that?

The response to mestinon is unpredictable in MuSK.

Some patients respond to it like AchR MG.

Others have increased weakness+fasciculations.

Others have both, but in a very hectic manner. The same dose at the same time of the day can lead to significant improvement, severe worsening or no response.
You can have fasiculations with improved muscle strength or with severe weakness.

The reason for that is (probably) that MuSK is a very complex protein. The antibodies cause: presynaptic abnormalities (less release of achetyl-choline like LEMS), post-synaptic abnormalities and also to complicate things even more- decreased activity of the acetyl-choline esterase (which is a mestinon like effect).

The balance between those effects can change, with activity and probably other factors. and thus the response to mestinon will change accordingly.
(those are my explanations).

Severe worsening with steroids may also fit. ( MuSK MG tend to have more myopathic response to steroids).

Were you ever treated with PLEX? IVIG is less effective in MuSK.

Also, did anyone suggest that you receive non-invasive respiratory support after having so many crises.