I would have to say you must look at intent. Although the first duchy changing allegiance is not and was not against the rules because there was still another duchy. The intent was that the whole realm was merging and what was left was to be forsaken to the rogues, thereby eliminating Solaria.
This action is not an exploit of a bug. However IMHO, it is an exploit of the game code not being able to take into consideration the intent of the game. It has been said many times that friendly mergers are not allowed.
An example. On AT you have Strombran. It is essentially a one duchy realm with a second duchy in Clyderee which is only one region. Let's say that Strombran's duchy joined CE so that they could fight a war on Easton and would not have to travel as far for banking. The realm of Strombran is still there, but it really isn't.
You clearly didn't get the intent of the first Duchy Changing. Duchies changing allegiance does not amount to a friendly realm merger. There was nothing friendly about Sun Hall leaving Solaria at all.
