Changeling not chosen a Fae yes, Fix became the Summer Knight a Mantle for a mortal, despite being a Changeling.
Some Changelings have had powerful access amongst the Fae- Sarissa for example would be a potential candidate but she would have been forced to drop the coin when the Summer Mantle went to her removing her mortality. Mab couldn’t have engineered this could she?, knowing Sarissa was a Denarian she manipulated Maeve into killing Lily so the Summer Mantle would force the coin off of her, and put Sarissa in Summer away from defending the Gates in the knowledge she had Molly as a back up Winter Lady?
Actually, yes that sounds exactly like Mab.
If Sarissa was a Denarian picking the coin up to defend herself against Winter, would she have been kumori? I think so. Kumori healed that chap in Deadbeat, she would have known Lea would want the Athame in Grave Peril. She would know Arctic Tor for the attack on it to rescue Lea. Everyone tried to fit Kumori against female characters introduced before Dead Beat, we don’t meet Sarissa (which is a weapon, a Macedonian Pike) until much later on in Cold Days. She gave Mab the idea for the Frozen Ring for Maggie, pure evil.
It’s a Moot point on a half red vampire, you would have to find one first, you might have a former red of advanced age saved by his coin from immediate senescence.
Besides with Goodman Grey why bother with a blackened Denarius when a Peace Dollar would be just as effective at binding him.