I have no idea why a king, duke, or even baron would risk his position with no gain from it. If someone wants to duel for the position they should have to have a risk too not just the one being challenged. Also I doubt your rebellions, secessions and wars theory will really happen, the only thing I can see is the judge fining the title holder until he gives it up and even then its unfair because the one who dueled still has no way to gurantee they get because appointing has to be agreed upon by the duke and elections use either the lords or all nobles. Again its lopsided as one would risk a lot by challenging someone to a duel for the title in the first place and then they don't get anything from it except a reputation of being aggresive?