Have a private conversation with your mother.

You smile at the Duke while saying, "Now, due to your future engagement, there are some things I discuss with the ex-queen in private. If you don't mind," You point him towards the door.

However, the Queen hugs the Duke's arm. "Whatever you have to say to me, you can say to my future betrothed."

You glance to the Arche Duke as you say, "With all due respect to your future Huband, the Arche Duke, but he just defected to our nation, and what we have to discuss deals with our countries secrets."

Hearing your explanation, The Arche Duke kisses your mother's hand. "It is fine, my greatest of loves; it is wise of a King to hide his secret from those that are new to his presences. Thus I will be patiently waiting in our carriage to cart you away as not to create any more tension than I already have." He gives her hand a second kiss before blowing to you and promptly exiting the hall.