For some reason the mantle needs a host. Probably to add intelligence to the mix because the mantle on its own does not seem that smart.
I'd say not just intelligence but a physical form -- maybe any ability to act at all. The mantle by itself, IMO, is essentially a thaumaturgic spell letting the person draw on "Winter" (permanent/not eroded by sunrise). I don't think it's even as individualized as White Court Vampires' Hunger spirits -- it seems like Harry, Maeve, and Mab are drawing on the
same "Winter" source, though it affects them in different ways.
The mantle is a power cord, or at most a battery - it can't actually accomplish anything unless it's powering a device.
The mantle can not exist on its own so it can not simply use a template and be done with it
Well, maybe template is the wrong word, but the mantle does
something to let Immortals come back to life when killed.
Bob talks about how eventually the Winter/Summer Lady becomes indistinguishable from the previous one, so there must be an "archetypal Winter/Summer Lady" that the Mantle tries to push its host into becoming.
What I'm wondering is, during the recovery-from-death process, does the mantle restore the Winter/Summer Lady as she was immediately before death, or does the process transform her into the "archetypal Winter/Summer Lady"?
I think that if/when the host changes that is purely because of the choices the host makes. These choices are under the mantles influence of course but that does not change how these things work.
For the Winter/Summer Knights definitely -- Uriel in GS and Mother Summer in CD make that pretty clear, IMO -- but the Knights aren't Immortals.
I think the case is a bit murkier for the Ladies. The clarifying WoJ about whether Molly lost her soul isn't much help, since it seems to be referring to loss of soul in the sense of damnation, not in the sense of being transformed into something that doesn't have a human-type soul or afterlife at all.
We have a POV from post-transformation Molly in Cold Case, so it's clear that she didn't become thoroughly transformed/soulless simply by becoming Winter Lady. I don't know that we can rule out Immortal post-death "restoration from saved game" changing that, though.