Bny806,
You sound like you may have LEMS , too. The main thing that makes it difficult for me to breathe when standing is that my chest wall and diaphragm feel heavy. It almost feels as ifmi am collapsing in on myself, if that makes sense. This is promptly relieved if I sit in a reclining position or lay down. Sitting in an upright chair is difficult, but not as bad as standing still. Walking is easier, because I think the muscle use is shifted around instead of calling on the same muscles over and over in standing.
Yes, I do have muscle wasting, especially in my left thumb and wrist, but now mostly to some degree all over. The thumb and wrist atrophy is what brought me to my current neurologist for a second opinion because my orthopedist wanted to do four surgeries on my arm and hand to try to correct this.

And yes, I do get deep muscle pain and cramps. When I was on plasmapheresis and the calcium drip ran out, I had buns and abs of steel, couldn't talk and could not move my diaphragm to breathe. I get really weird spasms in my jaws that hurt and the rest of my face gets contorted when I am really weak. Sometimes at my worst if I look in a mirror, I do not recognize myself!
And, yes sitting in a car, especially upright, is very uncomfortable. I have a squishy travel pillow, the kind with the tiny foam beads to help hold up my head and I have to recline the seat if I can.
And,yes pain with just eye movement, too. Although eye pain is very minimal and only occasional since plasmapheresis. Also, with the eye pain, extremely blurred vision. This is better too after plasmapheresis.
Hope you can get diagnosed soon and on your way to feeling better, because this is no way to live.