Interesting. Why did Filch not recognize Harry when he spotted him when they went to the duel that never happened? I assumed because he is a squib that he saw Harry as himself. Of course that didn't explain why he didn't recognize him. If he knows all the students, then he would have seen Harriet at some point. Maybe there is something else at work other than what I guessed.
Harry seeing Harriet hugging him in the mirror begs a question. Is that reviewer correct that said Harry had switched places with his female counterpart in another dimension? Of course, that doesn't explain why the Dursleys still saw Harry, and not Harriet. It is a mystery.
Author's response
Thanks for the multiple reviews. I like your insights - good way of thinking about it!