He's a cop who works in a legal system designed by hell, his ability to interrogate her is different than ours from a legal point of view. Permanent injury may be beyond what he can technically authorize us to do.He just violated her mind. If there is a law for that, and considering that he just offered us to let it do it, then he would be breaking it by inviting us to break it.
@DragonParadox, do we know of any legal problems to use greater curses to interrogate?
On the summoning front, an Astral Deva is only CR 14, so could we summon one to detect lies for us? Since summons are just copies of the idea of a creature it's not like we need to worry about getting someone local involved, and it's weaker than the monolith we just summoned. Not sure if it's something we can actually grab though. We could go with a Couatl with the appropriate spells prepared since it's definitely on the list, but that's a bit too exotic for this identity in my opinion.