I'm not a doctor but I'm just skeptical that such severe, constant breathing difficulties would be caused by Thomsen MC, or even the more supposedly severe, Becker MC.
I think that the one time I fell and had an episode of complete rigid paralysis, my rib muscles might have locked up to where I couldn't inhale, but I was able to breath again after I hit the floor and my muscles relaxed.
The one breathing problem I do seem to have on occasion, which I previously mentioned, my sister seems to have as well...in fact she has been complaining about it since childhood, but she's never exhibited any signs of MC, not even when pregnant.
I only know of two situations where severe breathing difficulty has been documented with MC. Certain medications administered for intubation can cause a masseter spasm and muscle rigidity in those with MC, which can lead to secondary breathing difficulties. And the other exception I know of is, there were some myotonic kittens a while back who's owner claimed died of breathing difficulties.
The kittens are an unusual case though. They were severely affected...more so than the goats, and this was probably compounded by the energy and impulsiveness of the kittens. While the kittens obviously had a myotonic condition, the cause my have been different than the cause of MC in humans. I'm not sure if they had the warm up effect and I think they probably tended to lock up multiple times a minute because it seemed to happen every time they went to move suddenly, which was most of the time they tried to move.
I wouldn't discount that her problems could be from the MC, but I would keep looking for an alternate cause as well. I would also ask the doctors, if she didn't have MC, what would they suspect?