She may have felt Titania was infected, but that does not explain why she piled everyone in winter on the border of summer. That action effectively prevented either from participating in the fighting.
Mab could have achieved the same objective by having summer and winter bring equal forces to the battlefield while still keeping back a substantial force to defend winter. Winter is still protected from betrayal by summer, but she also targets the red court which attacked her borders -- and is clearly allied with outsiders. Yet she did not do this. She had some clear reason to keep summer and winter out of the fighting. A reason important enough that she was willing to let the white council fall.
Yet this compelling reason did not prevent her from standing by while Dresden brough summer fire to the heart of winter. No way that little butterfly of summer power was missed by Mab. Which had a huge effect on her borders.
Which means she
a) was testing summer's reaction
b) was compelled to do what she did (block summer and winter from going to war) by some promise or debt.
c) she was testing Maeve
"a" is a pretty weak reason as it is a pretty unreliable test. Even if summer attacked winter in full force, it would not accomplish much. So a corrupted winter would do little. "C" is a possibility especially as Small Favor was after this attack, but what about Maeve's behavior would reveal she was infected to Mab. After all, if Mab was insane, then Maeve's actions would have been loyal to Winter.
B is the easiest explanation - but that is still a pretty big favor.
Not trying to shore up my failing hypothesis here, but what if it was also about the White Council traitor? At the end of the book, Ebenezer points out that the Reds were able to hit the White Council (coinciding right around the attack on Arctis Tor) at their Baby Warden camp, presumably through the NeverNever. He mentions that only a handful of people knew where it was. It might've been motivated by an attempt to narrow down the suspect pool, or confirm an existing one, and could've been part of an agreement between Mab and the Gatekeeper.
Note, it's only that Ebenezer fails to consider the paperwork that Peabody wasn't on the list; he clearly
did know everything, he was just beneath notice, like an evil Ehren.
