Sorry I'm at work so haven't time to read entire thread but our stories are very similar so I thought I'd reply quickly. I have had same dilemma only my RA was seronegative and non erosive and went away after a few years on MTX and Hydrocochloraquine. The SFN continued to rage however but all immunology was negative following 3 years in dmards and still on low dose steroids.

Finally my ANA swung positive three months ago - same number as yours but nucleolar pattern pointing more to Scleroderma - as do some of my symptoms. My new rheum organised for me to have US of parotids and lip biopsy. US was negative for inflamed parotids but lip biopsy came up strongly positive for Sjogrens. It doesn't matter if you have primary or secondary SS - it can and often does cause SFN and doesn't always present as dry eyes or mouth first. Worth a thought? I've just posted my symptom list and questions for neurologist - who is a bit of an Antiller the Hun so I need armour plating for her! If you read my list and recognise symptoms then perhaps you might consider getting lip biopsy done to rule Sjogrens in or out properly? It worked for me at any rate.

Ps my IgA is always raised and I think this goes with high inflammation/ proteins. Last time checked my IgG was also raised, as was compliment - and it was this that made my new rheumy think I might have Sjogrens as my primary disease and cause of SFN.