I think it's real Arya, but not because of Bran at all. Bran could have any number of reasons for giving the dagger to the waif. His former attachment to Arya wouldn't affect that. He could conceivably give it to her for any number of reasons. He can see outcomes no one else can, so it's hard for us to "logic" out his reasoning, and he has no more emotional attachments that would affect his decisions.
I just don't think it's the waif because A) she was too-immediately adept at interacting with Sansa in very familiar ways, and B) Sansa knew where to find her even though the method was through Sansa's intimate knowledge of the old Arya's past and personality, not to mention that the waif would not know to go stand in front of Ned's tomb and wait for Sansa.
Also, the waif would not be acting in ways so contrary to the girl Arya was when she met her. She would act more like younger Arya so as to not confound her family. She is acting like current Arya, and if she were as cunning as some are giving her credit for, she wouldn't do that. That would make her a horrible Faceless Man.
Plus Walder Frey stuff.
Fun theory, but it would absolutely suck for logical reasons and be 100% unbelievable for the above reasons.
All that said, something I noticed re-watching. Bran's dead-eyed zombie demeanor has a purpose beyond merely conveying his nigh omniscience: it lets us know when he is doing things that really matter by showing any questioning or emotion at all.
There is only one point in the episode where he shows any sign of being moved or otherwise affected by a decision he is making: when he hands the dagger to Arya. That interaction elicited a tiny flicker of a reaction. I would not have noticed it were he not a zombie at all other times.
