Would Scott have died if a non-werewolf had killed Peter?

No,because who ever kills the Alpha cures him,not kills him

nope

No. Scott's life does not hinge on whether a human or a werewolf kills him. There was a possibility that he could have cured himself of his lycanthropy had he been the one to kill the alpha who bit him, but as we already know he wasn't the one who killed Peter. Other than a new werewolf becoming the alpha, nothing happens to the other werewolves when the alpha is killed by a human.