It took six months or more from first testing for my antibodies to show up positive. Everything was negative in the beginning. Antibody levels wax and wane and don't necessarily correlate to level of disability.
No, Ivig didn't work for me, otherwise I'd be getting that. The symptoms you describe are me exactly, when I am needing plasmapheresis again. That's a scary place, since one little event can tip you into shortness of breath and all that goes with it.
You may be able to get it through your arm veins. I tried it and my weak, slack veins would not support the pressures of the machine when it tried to pull blood through them. So I had to use a tunneled catheter until I could get my dual vortex implanted catheter placed. My very first treatment, I had a temporary catheter placed, but had to stay in the hospital and have it pulled when I left. This would get you out of a crisis if, God forbid, you have one.

Someone was questioning my diagnosis, not sure who, was the reason I had the sfemg last month. Not sure if it was the powers that be at the hospital or the company that provides Dap, or possibly the insurance company. Initially, my doctor does not like to do plasmapheresis on a scheduled basis, but my body seems to require it.
It couldn't hurt for you to write your own appeal. And I'd ask the question about whether they would prefer that you go into the ER to get treatment when your breathing becomes compromised due to lack of outpatient treatment.
I would also ask that the antibodies be retested because, like me, lack of a super solid diagnosis can come back to haunt you.

I'd also ask how many LEMS patients your doctor has seen. They are few and far between. And those qualified to prescribe dap even fewer.
Who do you see? You may have to switch up doctors to find one more qualified.