
The assumption I'm making is that the game isn't checking if the Marquess Nation Mission was completed and is just assuming since the player is ranked as a governor, that the mission was completed. This was likely overlooked as it was assumed that players would complete nation missions.
Suggestion: Players who are appointed governor who haven't completed the Marquess Nation Mission should have a different title such as "Acting Governor". Thus if they are removed as governor, they return to the rank they achieved via nation missions, if any at all, and prevents the circumvention of nation missions to become king/queen or a member of the council. If the appointed governor, completes or had already completed the Marquess Nation Mission, then their title would list as just Governor.