As much as it is painful to see Navier suffer from these trashes, i know the end result will pay everything off. I think the duke is being the 2nd best wingman for king heinley, manipulating trashta in becoming the empress, Navier and Shovieshit could divorce, and then Navier can become the queen of the Western kingdom.
It was like hitting multiple birds with one stone. The Western kindom will have Navier and Heinley as thire intelligent and kind queen and king, while the Eastern will suffer from their trash empress and emperor.