Success of Begian revolt was merely due to a cluster of bad decisions by the Royal family. It could be easily repelled before it came to a real revolt and independence. No need for other power to intervene and no need for other powers to keep a Benelux kingdom together. Only more common sense and an equal division of power between North and South. After 1830 troubles this could be done by simple economic reality. The South was industrialising rapidly and with this an increase in economic growth. It was a matter of time when the economic power of the South will shift the balance of power.
Mid size Power, according to 19th century view.
Basically the power and influence will be, Netherlands + Belgium = 1+1=3
The South Netherlands (Belgium) was one of the first industrialized nations of Europe. The emerging industrialised (textile) industry of the cities of Ghent and Aalst benefit enormously of the overseas possessions the nation had, inherited form the Dutch Republic.
It would probably will run the first railroad on the European continent, most lieklely enxpand it into it most densed network of the word by the end of the 19th century (as did Belgium did)
The overseas possessions left to the new kingdom after the London treaty of 1814 will most likely provide the base of a colonial empire. This will include West Africa, Suriname and of course Indonesia. It is posible that a new Southern African colony will be establised in OTL Maputo connecting ith the Boer Republics. The Boer Republic could be protectorates of this Benelux kingdom and might be act as a stage for further colonial expansion into OTL North and South Rodesia or parts of it and if (TL Katanga or parts of it. (OTL African borders are made up in Berlin) This is possible due to the growth of the economy due to the rapid industrialisation and need for raw material and markets. OTL Belgian Congo will NOT exsist and will be part of French, Portuguese and British colonial empires and Katanga could be Dutch - Boer Republics.
With or with out African possession the presence of Dutch Indonesia and a very well developed heavy iron industry will make it very likely the the Benelux Kingdom will build up a battleship fleet as self respecting powers did in the 19th century.
The Royal Navy will be have some significance. The Army how ever will be more or less as the Dutch and Belgian 19th version only larger with a large emphasis on defence, in other words large fortificated defense lines. How ever the military will be involved in colonial wars as it was in OTL.
Diplomatically it will be remain neutral but it will be a power to be recon with. Prussia, Austria or France will try to get it in their camp or make sure it is neutral in any conflict. Britain will regard it as a major economic competitor.
Internally it would be heavily divided between Catholics and Protestants who bith will despise the Liberals, who eventually will form the governments for most of the 19th century as they did in both countries OTL.
Language will be no issue, French is the language of the upper class, but, as street language will be limited to the areas around Namur, Charleroi and part of Hainaut. There will be no suppression of language as in OTL Belgium. Since there will be a large migration from Flanders and other Northern Provinces towards the industries and mines of the South. The migrants of the North will dramatically increase the Protestant communities in the South. The Protestant migrants will use the Dutch as church langues as might be the Catholics from the North and so reduce the French as language.