It’s the company car all over again, attack Harry’s home Mab will ignore, but an attack on a Winter Garrison she will not. Whoever (Merlin) tried to bomb the Castle will have to answer to her at some point and that gives Harry as Winter Knight the capacity to challenge to a duel whoever organised it (Merlin) with the full power of Winter behind him. As a Vassal of Winter on Winter Business Harry would be guaranteed unmolested access to the White Council and specifically the Merlin to issue the challenge. It’s the only way he could get one on one with the Merlin and we know from the Gatekeeper this is going to happen in the future.
If Harry has any sense he would ask Eldest Gruff to be his second.
An attack on a Winter Embassy is an attack on Mab and will not be tolerated.
Eldest gruff is Summer. Titania likely blocks it. Scary Sidhe Lady is Winter. Mab is likely fine with Lea going, and she outclasses anything the WC can muster, even Eb.
Two spit-in-the-face seconds choices:
1) If the duel is considered Winter-WC : Former WC apprentice Molly
2) If the duel is considered personal: Grandpa Eb. Also takes away Langtry's best battle mage.
The one that spooks everyone on both sides? Rashid.
(Rashid likely turns it down. Uriel turns it down, but even if he didn't, they would know there would be no treachery. Play by the rules and you're safe. Vadderung would turn it down, then show up with a horn of mead and bowl of popcorn to watch. Possibly with the Erlking and Gruff. )