Yep, I read the same passage from Cold Days in formulating my theory, and it is consistent with it.
Again Lea being on ice in Dead Beat is a given, but not that Maeve is infected at that point. Lea is still infected by Proven Guilty and the attack on Arctis Tor to “free her” is supposed to have failed, but it is only presumed that this was to “free” Lea. It may have been the point of infection for Maeve as well as to test the level of protection of Demonreach’s backdoor.
The invaders were not portalling into Arctis Tor, they broke in via the Front Gate, they would have gone to a marshalling point in Winter not protected and from there and travelled by foot. Part of Nemfected Leas purpose may have been to be discovered and inprisoned, either confirming or re-making the backdoor to Demonreach.
Pell’s theatre may have been a natural congruency to the part of Winter where the phobo phages dwelt, it dealt in phantom images of fear, and had for quite some time. If so it would explain why Peabody travelled from Edinburgh to Chicago, he then only had to travel across town by cab to Pell’s theatre, come out near Arctis Tor to be met by Maeve who took him to Mab’s ice prison and the backdoor to Demonreach. Peabody simply didn’t have a more direct route, and this route had been surveyed by the Black Council in Proven Guilty.
Mab may have allowed the attack to confirm a few things, such as Maeve’s connivance, Denarian involvement with the Black Council, having this as Molly’s audition for a potential Winter Lady, and getting a further handle on Harry to be the Winter Knight. It may also have been allowed to lull the Black Council into a false sense of security.
Remember Nicky was owed a favour by Mab before and during Proven Guilty, so wouldn’t the attack by the Denarians have erased that? It confirmed Nicky wasn’t in control of all the Denarians to Mab, as the favour still stood until Skin Game. Harry further confirmed this in Battle Ground with Namshiel.