So I sat here over the last 2 days and am getting ready for another re-read of PG. Going into that, I want propose my theory.
First to answer the questions from the OP: AT was attacked by forces aligned with the Outsiders for the purpose of driving a wedge between Harry and Mab.
My theory in a bit more detail.
I am of the view that there is a lot of Fog of War in the DF. I think that a lot less is known about characters as they enter the story by our main actors than we often assume. In this case, Molly is a target of opportunity. Her magic is noticed by some agent of the Outsiders (and I have trouble organizing them) and a trap is laid around Splattercon. The idea is to kill Molly at AT and have Lea be the only being there to talk about what happens. Lea will claim that Mab did the killing or that Mab ordered Lea to kill Molly.
From there we have a kidnapping plot around Splattercon which goes rather badly for the Outsider forces. Eventually they do succeed and take Molly. Now, in my model Mab is occupied and Maeve is the highest level Winter Court member around. However it is done, Maeve betrays the defenders of AT and the battle is lost. Who is in the attacking forces does not matter too much, but it is headed by infected characters - Maeve, Eldest Fetch and a Denarian (presumably Thorned Namschiel). The idea is to kill all the defenders so that there is nobody to be a witness to future events. Eldest Fetch is designated to kill Molly. This would be done to have Harry see a senior member of the Winter Court do the killing. That is why Molly is not killed off screen.
In typical Harry fashion, he overcomes the Scarecrow and wins the day. At the same point (in a bit of confusion), Lea's infection is revealed to Harry. Mab assists Team Harry's exit via time manipulation (slowing the pursuers) and winks at him to show Harry that she has seen what he has done and approves of it.
So, I want to point out here what is supposed to happen again from the Outsider side's perspective. Molly is killed, hurting Harry's relationship with Michael. On top of that, Molly's death would be placed squarely at the feet of the Winter Court. Just FYI, this might drive Harry to take up the Coin and take him out of Mab's WK list forever. Molly's death at AT would have definitely put an end to any potential Mab/Harry hookup. Imagine the following line:
Lea: "Dear Child, mine Queen has gone Mab. She has killed your friend's daughter and imprisoned me to keep me from warning you. Maeve can help you protect your friends. Seek her out and tell her we spoke."
Edit: My view is this is the place and time where Mab found out that Maeve was infected.