WRT Belgium - perhaps France is less understanding about being unable to put the Duke of Nemours on the Belgian throne. When TTL's Ten Day's Campaign occurs, the Belgians seek French assistance, but they refuse. The Dutch, under Willem, Prince of Orange, occupy Flanders, Antwerp and Limburg, encamping at Leuven outside Brussels.
1830s; This crisis, coupled with the economic pressure of the loss of overseas of trade, triggers unrest in Wallonia - particularly directed against the government of Leopold I in Brussels. Poor harvests in Flanders lead to the Dutch offering aid, giving rise to pro-Dutch sentiment. Leopold I attempts to exert more monarchical power, particular around control of the military and in religious affairs, favouring liberals and angering Catholics in Wallonia. This anger bubbles over and results in the Walloon Revolt against the Belgian government, leading to the French intervening and occupying Hainaut, Liege, Namur, Luxembourg and Liege.
The 1839 Treaty of London recognises the territorial divisions of the status quo as it exists in Belgium. The duchies of Flanders and Limburg are created and granted to Willem, Prince of Orange. They remain semi-separate territories until the 1867 Treaty of London, confirms their status as integral Dutch territory - in the modern day they are constituent countries.
Luxembourg emerges as OTL. France seeks to annex the "occupied" territories of Wallonia, however as in OTL, the Great Powers oppose France acquiring any territory from Belgium.
And then things get vague... Eupen-Malmedy (ITTL retained by the Netherlands rather than Prussia) is created as a principality? It follows a similar history of Luxembourg, eventually transforms into Amikejo?
The Leiningens, having been promised the Walloon throne by Prussia, are compensated with Eupen-Malmedy (their British connections help), and reminiscent to OTL, has a close relationship with Prussia. Eupen-Malmedy maintains its relations with both the Netherlands and Prussia, but has an economic history related to Moresnet (the mine being exhaused in 1885 OTL) that ends up resulting in socialist revolution, which overthrows the monarchy?
The next few years for Eupen-Malmedy ends up chaotic under the newly established republic, and eventually results in the return of the monarchy (or at least, a weird variant of it?) and the transformation into Amikejo in the 1900s?
Wallonia established? France asserts historic claims to Hainaut, but Great Powers having none of it. Compromise with French protectorate over Hainaut (under cadet royal branch). Wallonia guaranteed neutrality under a German-descended monarch. Belgium dissolved and restricted to Brabant under Leopold I.
Now with Wallonia and Hainaut - my thinking is that the division is very much artificially imposed by the Great Powers to prevent France making any gains. Now Wallonia might gradually settle into neutrality, seeing a Walloon revival, but Hainault I imagine being quite vehemently French. The Duke of Nemours gets Wallonia, while the Arenbergs are installed in Hainaut.
However, when the 1848 Revolutions break out, the French monarchy collapses as OTL. TTL's Belgian legion attempts to begin a revolution in the Belgian states, but fails. Follow this with France's loss to Prussia, there is reduced appetite for unification. Coupled with improved economic relations with Brabant and the Netherlands, this pulls Hainaut gradually out the French sphere (for now). By the time reunification is next a viable option its 1919, and Hainaut has had nearly 80 years developing its own identity. Hainaut had the coal industry, which would have given it significant economic power itself, especially providing the resources for the industrial revolution in Wallonia. I can’t see any power allowing another to control that resource. Actually a lot of Belgium's early industry was in Liege and along the Meuse, although coal mining was prominent in the Borinage in Hainaut. So Wallonia would still be economically viable without Hainaut, it is has it's own substantial coal fields and industrial centre ...the continental Industrial Revolution may well be slower to take off however.
The Great Powers, perhaps now realising the importance of coal and the abundant resources in the region, don't like the idea of a French-aligned state controlling both Wallonia and Hainaut. France had reasserted a claim to Hainaut form the historic county, and now the Great Powers use that against them by using this historical pretence to separate Hainaut from Wallonia. The Arenbergs, a Belgian noble family with ties to Hainaut, are made king there, while in Wallonia the Duke of Nemours gets the throne.
Which leaves Brabant... I imagine being a rump Belgian state claiming the rest until reapproachment in the 1870s (seeing the Franco-Prussian War almost spill over into the Low Countries) Perhaps Leopold, now only ruling a smaller centralised state, favours the liberals more, maybe even trying to amass more political power. Could he be forced to abdicate in favour of his teenage heir in the 1848 revolutions?