Are you familiar with Plato's Theory of Forms? And related, The Problem of Universals? Type-Token distinction?
Your ideas/theories sort of remind me of all that, and my own observations that the DFverse and Mantles share a similarity with those related ideas.
The Theory of Forms, basically, is the idea that somewhere outside/above this realm, there is a place with the perfect Form of any given thing. Everything in our universe is like, an imperfect shadow or mime of a Form. So, say there are apples in our world, all kinds of apples, well somewhere in the realm of Forms, there is the most appley Apple that ever appled, and all the apples in our universe are just mimicking the apple Form, and while we as humans can never truly Know the true essence of the Apple, we can learn about the Apple by studying the various apples. It could, perhaps be that there is a Fruit form, that all fruits are mimics of. So the same is there the Form of a Circle, a Straight Line, Thinness, Fatness, Justice and Peace, and so on.
The Forms are perfect and immutable. It is our shadow world that is transitory and changing.
The Problem of Universals concerns how many objects and things can share properties, and whether Properties are real things, or only mental constructs.
Type-Token distinction is simply the basic concept, or idea of something, vs an actual thing, e.g the Concept of a Bicycle as compared to a red beach cruiser or a blue mountain bike.
All sort of related concepts, but Plato was pretty insistent that Forms actually, literally existed somewhere, rather than just being logical patterns that both exist outside and independent of our minds but also in a way needing our minds to give them name/purpose/meaning.
So how that can apply to the Dresdenverse, I have thought that there could be the Form of certain beings/concepts, but rather than actual thoughtful beings in and of themselves, you have the Mantles, which are the blueprints or patterns that exist in our world - the purest that can exist on this plane, and those Mantles get taken up by beings. That's would be why the Mantles exert themselves over the wearers—they're trying to mold the imperfect malleable vessel to become as close to the Ideal as possible, thus destroying what the the wearer once was, but also in the midst of that struggle allowing the Form a fully actionable presence in this plane.
For Winter/Summer they are actually the same thing, in that one can't be without the other in any real sense. When it's winter in the Northern Hemisphere it's summer in the Southern (think birds migrating). It's always Summer, it's always Winter, and near the equator the two are less distinguishable. In our plane the two have been partially separated for whatever reason, but logically and spiritually they can't ever be fully separated or distinct. They can't inhabit the same body because they're the same and would reunite into whatever they were before, the reaction of which would just be too much for the mutable vessel. Other Mantles don't have this problem because they aren't inherently the same thing split up.
The Forms, being timeless, unalterable, and indestructible, is also in a sense impotent. They need to express themselves in our lower, but more malleable realm, to actually do anything.
I also recall in Changes Vadderung mentions that the Lords of the Outer Night had lost much of their power, both by being less worshiped and by the power of their blood being spread thin. Maybe they started having too many Tokens and not enough juice to power them all? And it seems to me like you had the Red King, the oldest Vampire went mad with his Bloodlust, he started becoming too close to his parent Form, and started losing the ability to act outside what he was, much like what the Winter Knight Mantle is trying to do to Harry. So the real power comes in staying in an in-between state— drawing on the Mantle/power of the Form but keep one's own mind/purposes. That seems consistent with what Mab wants in her Knight and in a way she seems to exhibit it herself (alternatingly wanting absolute control and subservience, but also setting up constant rivalry and opposition for herself).
The Higher beings, they have become tightly woven into their Mantles, their Forms, they can't really act outside what they are, they can't act quite as freely in our realm. That's why they manipulate humans, because the humans can still make the choices to win whatever game the higher ups are playing. I think Odin and people like him just found a way to maintain enough of their humanity/individuality to keep an element of choice, and sort of found a way to cheat the system.
