Wouldn't that just delay the revolution then? Basically every criticism the south had was valid and people would rebel for way less. The south made up over 60% of the population but was represented by only half the members of the assembly, and representation in all other parts of the government was just laughable. Not to mention the north using the south to pay back their debts, which were more than 12x higher than that of the south, or the free trade policies that provided the north with cheap industrial goods from Britain but undercut the industry already present in the south (which meant both southern factory workers and owners got screwed over simultaneously).
So you've got a bunch of problems, every single one of them being created by the north to the advantage of the north, and you can't solve them without simply treating the south fairly, which would be entirely to the advantage of the south instead. If people have a job they're less likely to revolt, but that means protectionist policies that make the north rely on the south. You can make government representation more fair without making it entirely proportional (so still not giving the south the majority), but making concessions means giving power which opens the door for future concessions. And not using the south as a cash cow would put enormous financial strain on the north.
And the "Arimetique Hollandaise" used to legitimize the constitution was downright insulting.
Generally, I agree, although I have to admit that Willem I tried to forge those two distinct parts into one "amalgaam". Some of his ideas like the canal Gent-Terneuzen (and others), Algemene maatschappij (societe generale), the trifecta of southern production/northern trade/ colonial keystone, the Nederlandse Handelsmaatschappij etc. It's weird how on paper that all makes sense but when you look at how he did it in reality, his bullheadedness and desire to do too much at the same time let to his complete and utter failure. Had he not attempted to reform education and enforced the dutch language at the same time, but had he attempted them sequentially, history might have looked quite different.
How would the development of the UKN have gone?
Far more industrialised for the north. The Maas would be far more important for shipping. Both North Brabant and Limburg would become more and more important instead of periphery regions in both states.
How would the French minority in the country have been integrated into the kingdom?
Slowly and with much difficulties. French was quite prestigious, so much that the UKN parliament used it instead of dutch. Willem I, in the age of nationalism, should have to focus on the eternal battle with the frenchman. Associating french with the revolutionary terror might help as well.
Would the Constitution of Thorbecke still be there?
I doubt it, since in a united kingdom that was succesful, the monarchy would stand much much stronger. Additionally, the southern ultramontane clergy might be convinced to support a protestant king instead of godless liberals.
Would the Netherlands have a larger colonial empire?
I'm fairly sure they would have. The loss of the south and the revolution lead to a bit of a national trauma for the Dutch, who turned themselves into a "humanitarian superpower", leading to things like The Hague conventions. No loss of the south means a generally more confident Netherlands, which might even try to contend on a European level. Any French-german war might see the dutch join on the german side to steal some colonies.
And as most important question: How would the political situation of the Kingdom develop?
It really depends on how strong the religious/political pillars would be and how tolerant the kingdom would be in general. I would think pretty positive and stable, as long as the first half century (1815-1855) was prosperous and peaceful.
