Speculation on the Slytherin/Ravenclaw issue: Quirrell is a double impostor. He's Voldemort possessing a Slytherin (name unknown) and pretending to be that Slytherin pretending to be a Ravenclaw named Quirinus Quirrell. Dumbledore knows about one level of the masquerade, and accepts the explanation that the Slytherin of unknown name is a private person. Quirinus Quirrell may be an entirely constructed identity, although that would make it less likely for him to have failed to remember some of the details of it under interrogation.