Welcome Manatee! I'm so sorry to hear about your mother. I've been coping with MG since the mid-70's, and I'm 60 now. I've only had one crisis, and that didn't quite reach the level of a full blown crisis which needed intubation. Fortunately, I was able to self-medicate in the intensive care unit with my liquid Mestinon, taking a little extra and letting it work sublingually every time I felt that my chest muscles were ready to give up.
I had to do some research on desaturated lungs, since I had never heard the term, and one web site I came across showed just how serious it is. Of course, the situation described was different since it had to do with depressurization at high altitudes and the effect on the body when the lungs desaturated. From the info given, death is described as being practically instantaneous. Here's the link, if you want it. http://yarchive.net/med/oxygen_suit.html
I don't know what mechanism would have been going on in your Mother's case, unless it was that her breathing was so weak she exhaled and couldn't inhale again. If that were the case, her having agreed to accept intubation and/or resucitation would probably have saved her life.
Why did your Mother choose to refuse intubation? Don't answer, if you would rather not.
Hugs,