I'm not sure they'd split the realm that much. Luxembourg at least had a history as an independent polity when it was made into a state in personal union with the rest of the Netherlands and Limburg, while being part of the Netherlands, was in the German Confederation as well IIRC. Austria and Prussia also had parts of their countries that were within the Confederation and parts that were not. I think it is more likely that we'd see a Limburg-type situation, where there's a sub-polity of the Netherlands that is part of the German Confederation.
Also, considering how Dutch is just one dialect of the German dialect continuum when we really get down to it and how Limburg historically had a dialect that was much more different from standard Dutch than it is today, why wouldn't a similar transition happen in the Northern Rhineland? Dialects of German in that area (Low Bergish and Meuse-Rhenish) aren't that different from traditional Limburgish.
When coal mining starts up in Wallonia, what are the odds that there might be a sizable influx of Rhinelanders to the area? Would Dutch be used as a common tongue between the Walloons and Rhenish?
I don't think Willem II would live in Cologne, as the city would be right on the border. Maastricht would make more sense - historically Dutch, in German-ish Limburg, and in a very central geographic position not that far from Wallonia.
Also, as part of all the horse-trading at Vienna, what about Prussia getting Nassau in exchange for the Netherlands getting North Rhineland?