QQ can't be polyjuced, which seemed to be the Auror's earlier assumption, because it would have worn off now he's dead. So given that David Munroe looks different from Quirell, the obvious next hypothosis is that he is a Metamorphmagus. But still, that is extremely rare, which is suspicious. But next, surely some people who knew the real Quirrel are going to turn up to his funeral, and start asking questions like "what do you mean he's a Slitherine?"