i think JKR sticks to "under Imperius", just to keep things simple, and i generally just anglicize the latin terms -- imperioed is most natural to me, but hell if i know anything about latin. ;) i remember at one point there was a kerfluffle over 'animagistera' (yeah, can't spell that one) being the proper term for a female animagus, but honestly, it just gets a little ridiculous. the average hogwarts student doesn't know anything about latin either, so it seems perfectly reasonable to just anglicize it, unless properly declining it serves some sort of purpose?