If Mr. Hat and Cloak turns out not to be Flamel, then Flamel is..... Hagrid? Filtch? Dumbledore? Ron? Unseen? If Hagrid had appeared in any scene not required by canon symmetry or Quirell's fake Parseltongue explanation then I would suspect him more. Dumbledore's fear of being a dark wizard would make sense, but he seems too young and had a family.