I think the simpler "conscious Horcrux inside Harry" theories are ruled out by the sorting hat: "I can tell you that there is definitely nothing like a ghost - mind, intelligence, memory, personality, or feelings - in your scar. Otherwise it would be participating in this conversation, being under my brim."

It is possible that the original, canon Harry has been completely replaced by Voldemort, or that the Horcrux has merged with Harry to form a new single personality. The sorting hat is also explicit that it does not remember previous students such as Tom Riddle as individuals. Therefore it would not notice if HJPEV were in fact Tom Riddle redux. However there's just one person in Harry, not two.