1. In Skin Game, Mab wanted Harry to get the weapons in Hades vault.
Harry got all but the grail, but Mab knew that she couldn’t count on Harry getting everything, so it pretty much worked out as she planned it to. The blade of the spear of Longinus seems especially important in the current context, as it can be used to kill an immortal.
2. In the short story Bombshells, after Molly diffused the Fomor bomb and survived the attack of the Fomor Lord, Lea told her, “…But the Fomor are betrayers. Ever have they been, ever will they be. The only question is what form their treachery will take. The svartalves had to be shown.” Specifically, Mr. Etri needed to learn about Fomor treachery.
So, Lea knew the Fomor were betrayers, which means that Mab knew the Fomor were betrayers; and finally, both Molly and Mr. Etri learned exactly how treacherous the Fomor can be.
This means there is no way that Mab, Molly and Mr. Etri didn’t know that a Fomor sucker punch was coming, though Etri may have been distracted by the assassination attempt. I think we can also add Lara to this list. Lara sent Thomas into the Fomor stronghold in Bombshells for the same reason Lea sent Molly; to break up a Fomor – Swartalf non-aggression treaty. Like Lea, Lara was convinced the Fomor were only trying to divide and conquer the other supernatural powers piecemeal. Plus, Thomas probably gave Lara the details of what happened that night or at least what he knew of them.
It's also likely that Odin was clued in to what was coming, because it’s almost a part of his job description to know these things. That leaves Marcone. He knew enough about the Fomor not to be fooled by their peace initiative, but whether he and Mab were in direct communication beforehand about a possible Fomor sucker punch is somewhat questionable. I think Mab would have simply expected Marcone to be aware of the possibility. Not that Mab would have expected Marcone to be able to stop the Fomor, but she would expect Marcone to be able to roll with the punch.
This does not mean any of the above-named characters knew the form this betrayal would take. As in Bombshells, after Molly asked Lea how she knew that Molly would find the bomb the Fomor Lord was intending to detonate, “Bomb?” She (Lea) shook her head. “I did not know what was happening in any specific sense. But the Fomor are betrayers,” which leads back to what I quoted near the beginning.
So, Mab and company might not have known about the “Eye of Balor,” but I bet that at least Mab did know that Ethni… whatever, was involved. We know that even Alfred sensed her presence when he said to Harry, “THE LAST TITAN IS ON THE MOVE!” I bet that for Mab, the whole point of the peace talks was to draw Ethniu out of hiding. Get the Titan to become overconfident and a bit reckless. As a bonus, Mab would learn which signatories to her accords would fight to uphold them. Separate the wheat from the chaff; or perhaps I should I say, separate those supernatural nations that would remain in Mab’s good graces and those Mab would go out of her way to throw to the wolves. The leader of the LaChaise clan has no clue what kind of mistake he has made.
Harry thinks that he’s acting on the fly, making it up as he goes along, but just like in Skin Game, Mab has set the game up for him. Harry just hasn’t figured that out yet.
PART TWO
None of what I said above tells us what was going on with Thomas’ alleged assassination attempt against Etri, but it suggests to me that Mab was involved in some way. Not that Mab planned to violate her own accords, but she was probably aware of what was going to happen and tried to play it to her own benefit.
I’m not certain of anything except that in some way, Mab, Lara and Marcone are working together. I don’t mean the three of them have gotten together to have planning sessions. Clearly Mab and Lara didn’t trade favors by accident. On Marcone’s end, he had to know the potential weakness the dumbwaiter created in the gym when he agreed to have it installed and Marcone knew that Karrin Murphy trained at that gym on almost a daily basis. So, without Marcone having to talk to either Lara or Harry, holding Thomas made it fairly easy for Lara; with Harry’s help guided by Karrin Murphy’s knowledge, to allow Thomas’ escape while allowing Marcone to escape blame for the escape. It’s as if Thomas disappeared from Marcone’s custody by magic!
There’s a larger game going on that we aren’t privy to yet. It not just about someone pressuring Thomas to kill Etri, but about the countermoves that Mab; with some help, has been making.
Now for a WAG, or two. Mab will succeed, but not in everything. Harry is going to screw up her plans in some major fashion. Mab is thinking of replacing Harry with Thomas. She’s more than a little miffed that Harry won’t bend to her will and Thomas has much more obvious weak spots for her to apply pressure to. Of course, this possibility will be obviated if Thomas become a KotC. Mab will be stuck with Harry as her Knight, at least until Halloween rolls around. "Many, many mantles are worn-or discarded-on Halloween night, wizard."