Unthinkable Wings

I think it's explained in the book that the Un-Man is NOT precisely Weston possessed; it's more like Weston's a zombie. After all, his transformation included his biting the neck off a brandy bottle and swallowing the broken glass. So I think (which is very convenient for Ransom, of course) that Weston has already been "engulfed" by the forces of Evil beyond hope of recall before the whole attempt to tempt The Lady begins.