We have confirmation in Small Favor that Nic had no clue that any of the Fallen (at least any in his court) were present at AT.  IIRC, Harry pretty much calls out Nic about the attack when they had the convo at the Shea and Nic blinked in shock and hsi manner did seem to change has his brain was going overdrive at who among the fallen would do it...and why.

So we can count Nic is not involved in any form with the attack in PG.

Next question is Thorny acting alone or in concert with other Fallen on this?

To add to more fuel to the plot is also Mab's comment that she thought Molly was better suited to Summer. 

She may have been directly/indirectly working with Titania to groom Molly for a role in the SUMMER court, Mab had just discovered her Winter Lady was infected, but no worries she has a spare daughter if she can't cure Maeve.  But if the Outsiders/Enemy has gotten THAT close to Mab then the current Summer Lady has GOT to go, Lily was in no way ready for the role she would need to play if the Enemy got further 'inside' so to speak.  Molly could have started to be groomed become the Summer Knight or Summer Lady at the Proven Guilty stage.

It was only after her actions in Changes, messing with Harry's mind, able to make the hard logical calls to 'get the job done' as opposed of letting her very strong emotions/feeling for Harry get in the way of it would have gotten Mab's attention.  By choosing Logic over her Heart Molly showed she had the chops to be a part of the Winter Court, and Mab tailored her plans accordingly.

The thing to remember about the Accords is there is the Letter of the Law only...there is no Spirit of the Law in the Accords, which is why exact words are so importiant to the supernatural.  Mab doesn't care what you do, as long as you follow the letter of the law in the Accords.

You break the LETTER of the law, and Mab puts you on her list of 'People to do Bad Things To'.  In Small Favor Nic and Co didn't just 'violate the rules of a duel' he frigging KIDNAPPED the Neutral Party requested to supervise the conflict and resolve it!  Then subjected said party to untold horrors and tortures.  Yeah, Nic didn't just kick a little sand in Mab's eye with that stunt, he pretty much hurled a huge ball of mud at her and dared her to come after him.

So Mab took her sweet time to line up her revenge, she knows that Nicklehead corrupted by Nemesis invaded her Stronghold, with the help of her corrupted DAUGHTER, as far as she cares Nic could be part of the 'Team Nemesis' and even if she's not.  Per the LETTER of the Accords he's the head of the Order ergo any crimes commited by the Corrupted Nickleheads are his crimes as well.  So she's got that beef with him as well.

Nic got very well bloodied in Skin Game, he lost allot of his previous advantages.  Mab most likely has sent word to the other Accords members that if Nic wasn't on the poison list before he is now!   But as others have said...he's alive and still has some resources and the grail so he'll be back.