All that being said, as I am typing this, I realize that this prison was established by Merlin in the beginning days of the White Council. The White Council mapped the ley lines. It is possible that the prisoners are the original source of the ley line. Merlin could have created the prison and the ley line developed as the prisoners were added or he could have created the prison on top of an existing ley line and the prisoners supercharged it with their body heat.

What if Merlin is a Mantle? And no not like the title in the White Council.... though in thinking that, maybe Merlin is a Mantle that was forgotten (on purpose or by accident) but though lore/history the White Councile adopted it a title.

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..