Fix probably has diplomatic immunity - for the killer - but it may be an act of war if he attacks the Warden - and vice versa. I would not be surprised if Fix toasts him, and then when Harry (or Carlos, or Bill Meyers) comes in, he says "OK, take me in, it's a Summer Court and White Council matter, as in the Accords. " Plus, as noted, he has no innate magic - he is using the mantle given to him. The Mantle is the weapon, much like the White Council has not said anything about Harry SHOOTING Corpsetaker to death - but not via magic. He used a weapon.
I'ma little surprised Harry can BE the Winter Knight. That's an agent of Winter. Not an employee. I can WORK for, say, a Canadian company, but I can't be an envoy of their government... I don't think I could get diplomatic immunity for them.