I get the idea, though, that Aslan's creative act in Narnia is inviolable. This is illustrated when Jadis throws the lamp-post at him. As in Christian theology, the idea that God did not create out of complusion or necessity and had no compelling influences upon whatsoever when he created, so it is with Aslan.
Augustine taught that evil has no nature in itself but is simply the perversion of good--a thing Lewis believed as well. Whatever damnage the White Witch did in Narnia, it would have happened after creation.