It's really sad to have to show up plans, strategies and RPs to defend some situation, exposing it to everyone... hoping that they will understand the difference about what is discussed here and what they can use in their own benefit in game.
To use the term realm merger is incorrect, the term should not be used and another term should be used in its place such as duchy merger.
And I fully agree with this. If you don't want to see it happening, instead of open cases and always untangle situations like this, even bringing to the table some kind of bug abuse, try to create some code to avoid entire duchies moving to another realms leaving just one city behind... or live with that, because even when it's considered illegal, fix it after it happened is a huge inconvenience for a lot of players involved, especially when they are unaware that they are doing something wrong. That's useless to show it in our Forums and open cases if the game continue allowing you to do it.
Invariably you will find someone who doesn't know the rule and will not do with bad intentions, but simple ignorance of a rule that exists just here, while the game itself doesn't prevent you from making this kind of decision.
