i know what you mean about lupin not being himself since the third book, especially because he had become the kind of character who you rely on for protection of everyone else, and wherever his name is mentioned, things were sure to work out fine.

as for the way harry asked him, i suppose something about the curse put on grimmauld place could tell if it were the real snape, after all.. later on in the book you discover he was in the house anyway.. crying over lily's picture where he took the second half of the letter.

all the same it was a very good book, i was expecting more about teddy lupin however!