Eh, it's old-school magic running up against new-school supers. Getting some weird edge-cases like Hench Wench feels... not plausible, exactly, but it doesn't break verisimilitude, at least for me. Theoretically, at least, she's using human contractual formalities to boost her inherent devil magic, which ties into her superpowers. By doing that, she is magically bound by the rules of contract law. So using contract law to undo her powers works- Arianna might also have been able to remove her powers by pointing out what several people already have, that a legal agreement for the purposes of illegal activities isn't exactly binding.