Some more information on this still unresolved issue.
I noticed that on the pop-up info on noble rival "Marriage" is one of the solutions.
Since I married my sims in CAS when creating them, I thought maybe they need an actual wedding ceremony (I do own Wedding Stories).
So I changed them in CAS to Partners. I had the man propse, she accepted, and I held a simple wedding ceremony where they excahnged vows.
I had hoped this would effect a change and they wouldn't be rivals anymore. But alas, it didn't change the Rival status.
The man's title WAS changed however, when he was only the partner and engaged, his title matched where he is in the noble career. Once they made their vows in the wedding ceremony, his title was changed back to King.
I would appreciate some feedback from the EA team. I am pretty sure this is NOT what is supposed to happen.
If Marriage is supposed to resolve the Rival status, that's a pretty huge glitch that it doesn't work. It is breaking a vital part of the story in the game mechanics for the Royalty & Legacy Pack.
Thank you for any help anyone can give!