I think he's "Wild Fae" in the sense of "not technically owing allegiance to either Summer or Winter Courts.  The Erlking is his own sovereign monarch.
That's the bit where I need to track the exact language, and context; they might have just been talking about field-command and battle arrays, not overall mystic politics.

Similarly for the opening night of _Peace Talks_, before Ethniu arrives:  was Erl there at the talks?  Did he have his own separate "contingent," or was he attached  to Mab's group?

No, I think I could be wrong, but Fae like Toot began as a "Wild Fae" as well but pledged his allegiance to Winter after Harry became Winter Knight.  It could be that the Erl King started out the same way and as he grew also made a choice and became a vassal of Winter, but that's a guess.

Yes; but the "Norse Pantheon" has no particular supremacy over any others; and thus Odin might be supreme in his hidden-knowledge vs. other Norse deities, he wouldn't have the same advantage vs. the Celtic pantheon (from which Ethniu derives).

I remember vaguely and have to go back and reread that Vadderung acted a bit detached, I don't think he expected to get sucked into it..

Eb and Odin, I recall.  Lea (and/or Mab, by proxy) too.
I don't recall any clear signs of Langtry being there...?   (there is a puppy on my lap, I cannot go get a copy of Changes to re-read the scenes at Chichen Itza).

Langtree is on it, he was mentioned if I remember correctly in Changes when they finally showed up at CI.. Eb was also part of it, but off the top of my head I cannot remember who else showed up.  I wouldn't be shocked if we find out that Morgan was part of it and that Rashid is part of it..