At the beginning of episode 21 of the season 11, the master from the gemology society told that the diamond found on the victim was a piece of Topkapi Dagger which was given as a gift from Mehmet the First to Nadir Shah. It should be Mahmud the First.
A marquis (and his wife, a marquise) is nobility, or aristocracy, not royalty. He is properly addressed as Your Lordship (or Your Ladyship). While Aubrey may be being sarcastic in addressing the Marquis as Your Highness, other characters make the mistake as well.