I can't imagine this would be classed as "idiopathic' now that you have the confirmation of an immune mediated process. It seems obvious that you have an autoimmune process and there fore treatments for this should be considered.

Personally, I would push for the skin biopsy as it not only provides detailed information on the condition and percent of nerve fibers, but the test can be repeated at a future time (after treatment) to check progress in regards to both the condition and percent of fibers, but whether your treatment course is working.

It is a minimally invasive procedure that is tolerated well...even for repeat testing.

Due to the autoimmune process, IVIG should be considered. It has documented success with SFN due to an immune mediated process.

I'm not surprised by your rheumatologists disinterest...SFN is a neuro problem and not something he would treat. BUT, that being said, he should be in contact with your neuro for proper coordination of care since the SFN is autoimmune based.