Whether or not a creature is a Werewolf or a Wolf is checked only as combat damage is dealt. If the creature isn’t a Werewolf or a Wolf at that time, its combat damage will be prevented.

Moonmist causes any double-faced Human to transform, not just Werewolves.

Moonmist will prevent combat damage dealt by a creature that isn’t a Werewolf or a Wolf even if that creature wasn’t on the battlefield (or was a Werewolf or a Wolf) when Moonmist resolved.