Some Denarian (with hints that it was Namshiel) attacked Arctis Tor, but that's not the retaliation we're talking about. The Reds violated Mab's and Titania's territories, and Mab didn't allow any retaliation against them in PG.
True enough, but consider what her main goal was, it was Harry. Do you doubt that she didn't pull or couldn't pull strings so in the end Harry had no good options other than becoming her Knight in Changes? He went to Marcone for help, was turned down, he went to Vadderung, he was turned down and pointed in Mab's direction, even Uriel got into the act.. Once Harry consented, Lea was sent in to help, going through the Ways was how they were able to travel to C.I. and Harry proceeded to wipe out the Red Court.. I'd call that pretty good pay back, wouldn't you? Like I said, someone immortal or nearly so has a different sense of time that mere mortals, they can afford to wait until the timing and conditions are perfect.