Although Arawn works for Mab, he can't be a fey. He's like Odin, an Immortal with a capital I entity from before the universe existed/time began who has chosen to take on a mantle belonging to the Winter Court in return for greater ability to interact with the material world, just as taking on the mantle of a Celtic god allowed him to do back when that religion had worshippers.
I'm not sure whether when Gods have children, the mantle they're using at the time is the determining factor in their scion's powers.
Something to consider is that Immortals like deities can't be killed except on Halloween, although mantles can be transferred. Mortals can only ascend to Immortality on Halloween as well.
Thinking of why Molly smells of divinity, if the old gods predate the current universe, so may her Exaltation.
He is not Fae. Probably. However, he is Winter Court. Just like Knights are Court without being Fae.
Or how you have human lawyers allegedly sworn to Winter.
We dont see godchildren in canon, so who knows if its even possible.
The closest I can think of is Dracula son of Drakul, and it didnt seem to matter.
The Ankou was worshipped as a god, but that doesnt mean it was one.
They change how they present in a lot of fundamental ways, so I don't think it's totally out of the question that it does matter. We don't really see it at work in canon in a way that specifies one way or the other.
Mantles all have to follow some big rules, but the individual ones can have some variations.
For example, the knights' mantles can be moved around more freely, in part because they're weaker but also because they have a different purpose.
At the very least this whole sequence of events implies that Holt and corpsetaker think there is a loophole here of some kind that cheats the major rules, so I wouldn't take stuff like Halloween limit as exclusive fact. There could be edge cases or modifications from how the setting was crossed with exalted that change how things work.
Going to note that normally, the Sidhe Courts cannot interfere with mortals who do not already have dealings with them.
The Sidhe are twisty and can rules lawyer a lot of that, but it significantly circumscribes their ability to fuck with mortals.
Thats why the office of the Winter Knight and Summer Knight exists; people who are Winter/Summer but arent fae, and so can freely work their Courts will in mortal society. With the current Winter Knight hanging on a cross in Arctis Tor, being tortured for treason, Mab has limited options.
Which is why, I think, you see her using Dresden as an agent a lot more after the events of Summer Knight.
She's making do.
Losing the Elder Ankou, who is Winter, but has a preexisting portfolio that allows him to deal with mortals and similar entities that fall into his bailiwick like necromancers, mediums and the like would further circumscribe her available options at a time the Winter Knignt is out of play.
