Lester, you calf pain is likely from your POTS re: pooling of blood in the legs - consider compression stocking or water therapy / recumbent bike. Brain fog is also very common in POTS as well as IBS / Mast Cell disorders. There are known links now that many POTS patients also have hEDS or ehlers danlos syndrome - hypermobility.

EDS is a connective tissue disorder that usually presents as joint hypermobility and ligament laxity leading to micro tears and scar tissue formation. I personally have a long history of arm, hand and neck issues and whiplash. Numerous nerve compression issues from scar tissue.
I have been struggling myself with NTOS for about 5 years and been thru the gamut of all modalities and injections. My right arm and chest/neck are becoming disabled and weak. Can't even cut vegetables or prepare food.

Looking down triggers neck numbness. Raise my arms at all and it worsens. Dry needling, massage and cranial sacral therapy helpful but short term relief. Recently diagnosed with VTOS with subclavian vein compressed on abduction picked up with MR Neurography. To make matters worse, I fell about 12 years ago and had rotator cuff surgery on this side. Im making my rounds thru vascular TOS guys. They are afraid to touch me because of the ligament laxity. One surgeon is blaming the TOS on poor posture which no matter how much PT I do, the ligament laxity doesnt change.
Vagus nerve runs thru the neck and some are saying compressions in neck or vascular system cause POTS. Im not so sure about that although Im not dismissing it. My daughter has severe POTS, hEDS and IBS and all the symptoms you have. She had QSART test that diagnosed the small fiber neuropathy. This is common in POTS too. Not sure a skin biopsy would be of any value. Can't do anything for it anyway.