I thought Ahsa is planning to kill Stannis (she gave him a chance to just give Theon up, but well) and free Theon at the Weirwood. She has her two guys (or are there more? ...I'm a one time reader) and her suitor didn't seem so impressed with Stannis either, maybe he'll help. Of course most likely logical outcome is failure, but who cares, he is her little brother and she'll save him or die trying.
The knowledge about Ned and the ways of the north are surprising, but I agree she could have it from the Mormont girl. It is even a likely topic for conversation with people getting burned alive in Stannis' camp. People might be motivated to discuss the superior way of excecution in their culture.
Of course I hope Bran will get into the action, or already is - like in the raven, but for Asha to just be his sockpuppet with no agency of her own would be very dissapointing.