The problem is that Harry is representing one or both groups, and so is acting as proxy. While it's possible he is going all lone wolf, I think the Council and/or Mab would stop him from interfering. Mab would hardly need to try as I suspect him disobeying her would rob him of his mantle and paralyze him. In theory, Harry could act independently of the Council. In practice (especially considering the political maneuvering against him) he likely would give them all the excuse they need to either lock him up or execute him.
Mab wouldn't have any reason to command him not to defend his home if the Fomor attacked Marcone's territory, though. She wants them to fall, but so long as she doesn't order his action, she has a solid argument that her Knight doesn't have an Accorded obligation to let his home get trashed by a dispute between two other powers to preserve Winter's neutrality.
The Council don't have to be as strictly legalistic, granted. They'd be pissed if he intervened and dragged them into another open war while failing to kill the Empress ... but the Council are pretty remote, and if he fails in this, he'll be dead before he has to worry about their disapproval. On the other hand, if his worst weekend of the year ends up with a dead Titan that he reports to them as a fait accompli, what are they going to do about it? The Wardens have been in hard fighting against the Fomor for some years right now without a declaration of formal war ... bringing in the enemy leader's head would mend a lot of fences with pro-Harry young Wardens wavering over him joining Winter and being MIA for the Fomor fighting.
Besides, there's another out by which he can take Marcone's side, if that is the scenario. He still owes Vadderung a favour. If he trades that to Marcone to use on aiding the defense of Chicago, Harry isn't representing either Winter or the Council. He's paying off a debt as he's obligated to do.
