The Dutch leaders who eagerly welcomed William back wanted him to establish a monarchy. William wanted to establish a monarchy. Most everyone in Vienna favoured a monarchy. Going against that will be difficult. Perhaps the easiest way to do it would be for the Stadtholder and his family to die in a shipwreck as they flee the Netherlands. That way, there won't be such an obvious candidate to take the throne, and restoring the old republic with an elected Stadtholder (the more recent 'inherited' nature of the position being quietly dropped).
Agreed, as long as an Orange-Nassau is alive, the Netherlands will be turned into a monarchy, unless maybe if Willem I sided and cooperated with Napoleon for too long and the allies decide to get rid of him. Maybe in that case I could see the Republic restored. There is basicly no other claimant in the male line of the Orange-Nassau family and I doubt the allies would accept Louis Napoleon as king of the Netherlands. So a republic is basicly the only option.
Even without a stadholder, the new Dutch Republic can not be similar as the old Dutch Republic.That Republic had simply failed. It basicly was too corrupt and too unstable. Especialy if we add Belgium it would simply become weird. Would all of the Belgian lands become Generality lands? That would simply not work. Some sort of unitary state would need to becreated, with some sort of equal rights to catholics (and other christians and maybe even Jews). Probably the Batavian Republic or commonwealth would be used as a foundation of the new country, although both weren't terribly stable either (although having no Napoleon demanding all kinds of things would help.
So, how about the Belgians. Would they be happy? I think they would be more happy than they were in the Kingdom. At least there would be no Willem II to screw everything up. Also, I doubt this Netherlands would not be split in a northern and a southern part as the United Kingdom of the Netherlands was*. I do think there would still be tensions. I doubt the Flemish (and yes I do mean Flemish) upperclass would accept Dutch as a language. I doubt the Dutch protestant will accept the influence of the catholics (who are a majority now). So still lots of trouble ahead. The Dutch Republic will probably be relatively unstable and only a minor part of European politics. This will probably change once industrialisation start and the country becomes richer (money can solve a lot of problems).
*which reminds me, what happens to Luxemburg? A Personal Union is out. I doubt this Republic would be able to go toe on toe with Prusiia, so if Prussia still wants the forts of Luxemburg, I guess it would end up mostly Prussian.
