Rheinbund
Established Nation
If Willem III was so unpopular and psychopathic, I would doubt he abolished slavery out of goodness...but maybe you can surprise me by saying he did it because of that ;-)
Sorry. No. I can't. He mainly did it to at least become popular in the Caribean colonies and Surinam.
And for the nationality of our Royal House, it originates from Dillenburg in Germany. They are called Oranje-Nassau, but they descend from the branch Nassau-Dillenburg.
And indeed, Willem III didn't have a surviving son, and the Queens after him all married German noblemen, so also in that sense they are all Germans. But after our current King we will get a mixed German-Argentine Queen.