Well, side note but I personally think the key to the Mantle is that Mab did NOT give it to Harry, she gave it to Id-Harry (hence the Snowflake Badge) and so all the base urges and desires are not from Winter, they are from his own inner self that has been empowered by Winter and given a much louder voice. Makes sense to me that Mab would be able to sense/interact with him, and find him a more understanding/accepting (to her POV) collaborator
I tend to doubt that would be possible. Id-Harry is just Harry talking to himself, one side of him talking to another. I don't think Mab could give the mantle to
just id-Harry any more than she could give it to
just his left arm and not his right arm.
As for the dark impulses coming from Harry, I'm sure that's the case. The Winter Mantle brings out the host's 'wintery' impulses, ramps them up and amplifies them, but they don't come from nowhere. I think that's how it works with any host. A host who somehow lacked any 'wintery' urges at all, zero presence, probably would be unaffected by it, but he probably also couldn't be the Winter Knight in the first place.