I like your ideas- the White Witch's appearance and attitude really DOES scream "Major NPC in a WoD product", doesn't it?

An obvious faction for the White Witch (in Mage and Werewolf) is the Weaver- locking the world into an icy stasis forever seems right up the Weaver's alley. And that would explain why nobody has died form the winter either- that would be too much of a change.

Narnia-in-winter as an Umbral realm has lots of possibilities as well.