Hi and welcome to the Chiari forum.
I had this issue but it was while my incision was healing post op.....and I know it was due to the skin drying and pulling on the incision due to washing my hair....I was told to use a baby shampoo and not to wash it too fequently to help prevent the pulling as it could open the incision...BUT since it is a few years since your surgery I am not sure what is going on.....
I noticed you do have other related condtions,and it may be due to one of those that you have this unscratchable itch.
I will say my scar/incision site does bother me from time to time...but not like you described.