Diplopia, squeezing band of pressure in head, absent L posterior inferior cerebral artery. Chol/hdl calc 6.7. FH mutation. Could this cause stroke/tia?

No. Minor anatomic variants of the arteries in the back part of the brain are no big deal. Most of us pathologists think the "MTHF" mutation testing is subscience at best -- it's being pushed by one lab to lawyer-shy physicians and a majority of folks are told they "have a mutation" and need vitamins from the company. TIA / stroke come on suddenly and dramatically. I suspect this is migraine variant.