My idea for this is that Voldemort is on the back of Quirrell’s head, but then the Quirrellmort composite polyjuiced himself back into Quirrell, so that he doesn’t have the huge vulnerability of having a face on the back of his head.
(This explains the mystery of why he blocked the polyjuice detection spell that was cast at him at the ministry.)
My idea for this is that Voldemort is on the back of Quirrell’s head, but then the Quirrellmort composite polyjuiced himself back into Quirrell, so that he doesn’t have the huge vulnerability of having a face on the back of his head. (This explains the mystery of why he blocked the polyjuice detection spell that was cast at him at the ministry.)