A guilty verdict will also not remove ambiguity. It will just shift it to a different place. How many regions can flee the sinking ship? Can 3 of 5 regions run away? Only 2? If four go, who do you punish? The last one or two? All of them? What if the duke of a two-duchy realm runs, and then a couple lords follow? At what point do you say "enough is enough"? How many regions can be transferred between two peaceful realms before it's a merger? All but the last city? No more than 50%? 75%?
You cannot remove all of the ambiguity. But in the process of trying, you're going to confuse even more people. People in losing realms will now be forced to either fight it out to the bitter end, or just give up and walk away from everything they had. You've turned their already sucky situation into an even more sucky situation from which they have no good alternative. This removes another point of conflict, that could threaten to drag other realms into the war.
While I agree with your general premise of removing the ambiguity, I disagree in the way you have chosen to do that in this case. This is a fuzzy, inexact rule. That's why there have been so many cases involving it. (Even if some, like the Tara/Coria one, are completely absurd.) But you can't (well, you can, but you shouldn't) just ignore precedent just because you think it's a bit messy, and want to clean things up a bit. You will be completely destroying the intended spirit of the rule, as well as making the situatuion overall, IMO, even worse. Yes, there have been a few cases of this lately, but there many more that weren't cases that your new interpretation will drag into the forum here.
IMNSHO, the Magistrates should rule this case in line with prior cases that have already set the precedent, and then kick this rule back to Tom/the players for it to be debated and overhauled. That's the proper way to do it, rathed than legislating from the bench and completely changing the purpose and the historical interpretation of the rule.