Can the Archive help? Is her neutrality going to interfere?
Two other possibilities not mentioned: Toot-Toot and Harry's other Little Folk allies (possibly very useful for intelligence) and the Alphas (unless you're including them under Paranet members - but I doubt they will hide; they've been fighting the Fomor for a while).
-
With the Ways shut down, the Fomor attack is probably constrained to come out of Lake Michigan. Water is bad for mortal magic, but doesn't impede Fomor magic (as seen in Even Hand), so putting the wizards on boats is probably a bad idea.
So I'd station most of the Wardens and the more combative Senior Council members on the shore - within sight of the lake but not close enough for some big monster to grab them without leaving the water. Their job is to burn up the Fomor troops and counter and hopefully kill the Fomor sorcerers (like Mag from Even Hand). The Senior Council should take a shot at King Corb if they can. But they shouldn't engage Ethniu herself directly (if she shows up, pop into the Nevernever).
The Einherjar/Valkyries and the combat-competent White Court should largely be here too, with some of them (those equipped with pretty heavy modern weapons) as a first line of defense here too. (Though not mentioned in the original post, the Alphas probably also will be.) If the First Law keeps the Wardens from killing the Fomor-altered-human-servitors, then these forces will have to handle them.
The other White Council types should be doing intelligence, divinations, coordinating with the Fae and Svartalfar, etc.
While all those people and creatures deal with the Fomor army, Harry, Butters, and Sanya go hunt down Ethniu herself. They're probably the only ones present who could harm her without massive collateral damage. Either Faithsaber-mode Fidelacchius or the Spear of Destiny/Lance of Longinus/Athame is probably needed to draw the blood Harry needs to imprison Ethniu.
