...prominent csf space in the superior posterior fossa, unchanged. it is favored that this represents prominent subarachnoid space/normal variation rather than an arachnoid cyst. is this mega cistern magna? extra fluid? from a head injury?

Radiologists prefer to write up detail that confirms they have studied the films, They get criticized when they just say "normal" or "unchanged from previous studies". I suspect it just documents a normal variant than is nothing more,

Your description is most consistent with a congenital variant of no clinical consequence. This is not an outcome from "a head injury", and is NOT hydrocephalus. To be conclusive, contact the radiologist and discuss the interpretation directly.