If the attack on the nerves is from antibodies we still don't have tests for, yes, it may be helpful.

They will try it, and if it doesn't work, then you are no further behind.

Certain antibodies produced from bacteria and viruses (and perhaps vaccines), may not show up in standard autoimmune testing. Some autoimmune antibodies come and go and give confusing test results.

If your insurance pays for it, I'd give the IVIG a try. The medical community uses it much more often today, than in the past for many autoimmune inflammatory issues.

Autoimmune diseases may test sero-negative. Patients on myasthenia gravis forum here have tested sero-negative, and several posters here with Sjogren's syndrome have sero-negative results.

Mine spread in a matter of a few months also. I understand your distress. My blood work all comes back negative also but I have still been told that it is autoimmune. I am still searching for an answer for the neuroopathy but if I don't find it, I now ealize it doesn't matter.
What I am focusing on now is how I can get a better quality of life. That is not to say I will stop looking. I live in hope that if they find the cause they can stop it. hopeful