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.