Did they do a full set of MRI's of your back? If you had a significant back injury from the car accident, there is the potential for syringomyelia to develop down the road - which could cause these type of symptoms.

Some of the other things you describe also certainly suggest the potential for autoimmune issues.

The skin biopsy is such a tough thing. If it's positive, you know there's a problem but it doesn't tell you anything about what's causing it or how to treat it. And if it's negative, your still stuck with the pain and other symptoms - but don't know why.

Sounds like your doctors are going down the right path(s). Hopefully they can find a treatable cause. In the interim, spend some time looking around the forum for things that can help if you don't know the cause (diet, exercise, stress reduction, supplements).