Hi....yes, many with Chiari and related conditions have had issues not only with vit B12 but D and minerals Magnesium and potassium as well.
U r not going crazy....otherwise we all are....many of ur symptoms are the same as the rest of us , so u r not alone.
I am not familiar when boxing day is, so not sure how long this is, but the hot cold issues could indicate a syrinx, u would need a MRI of ur complete spine...cervical m thoracic and lumbar to rule one out...plus it helps rule out disk issues as well as tethered cord.
I know it is difficult to locate Drs well informed on this and ALL the related conditions, but we do have a list for Canada -
http://www.medhelp.org/health_pages/list?cid=186