My question / challenge is to get the Netherlands by 1950 to hold the current territory of the Netherlands, plus the Flemish-speaking portion of Belgium, plus Dunkirk. Belgium itself (the French-portion) should still be independent and have a coastline by the same date. What is the most plausible way to get such a thing?
On its own thisis already quite hard, but with an independent Belgium, with a coastline? borderline impossible.
My suggestion for the Netherlands including Flanders (I assume you also want Brussels) and Dunkirk would be a better 80 years war for the Netherlands, which ends with the Netherlands owning Antwerp, Bruges and Ostend, which allies with France who then devide te southern Netherlands. The Netherlands gaining the rest of Flanders (thus including Dunkirk), Brabant, Gelre and Limburg (or only Overmaas with Limburg itself going to France). At a later date the French and Dutch divide Liege, with the Netherlands getting the county of Loon and you more or less have your challenge, without an independent Belgium. Still the problem is keeping France out of Dunkirk and southern Flanders, as a coastline would probably worth more thana couple of hills; it should not be impossible though.
If you want an independent Belgium, you can't use any alliance with France, as France would want at least part of the southern Netherlands. There are various possibilities for the Dutch Republic to expand south, but it never realy cared for it. A buffer state always seemed to be more useful than annexing it properly, especialy as Antwerp could become a danger for the wealth of Amsterdam and the merchants of Amsterdam (which were basicly the ruling class) wanted to avoid that. So you need the Netherlands more expensionist. I think a surviving stadholder Willem II might do the trick. In the various wars the Netherlands got involved, it slowly expands south. Still getting all of the Dutch speaking Southern Netherlands is hard, certainly as after a while Spain (and OTL Austria) became allies against France and you generally don't annex lands of your allies. In this case Belgium remains Spanish (or Austrian or whatever) and later becomes independent. Still sea access for Belgium I can't see.
Other possibility. Waterloo: Napoleon defeats Wellington. The prince of Orange (the later king Willem II) dies on the battlefield. Napoleon itself is beaten by the Austrian and Russian armies that are approaching him. France is punished more harshly than OTL and (partly as compensation for the fallen prince), the Netherlands gains Dunkirk. The Belgian revolution happens, but as the prince of Orange is now death, he doesn't act like an idiot. His brother handles the situation more competently, but cannot avoid it completely. The Walloon provinces (Liege, Namur, Hainaut, Luxemburg) secceed and form Belgium. The problem with this scenario is South-Brabant, which most likely would also seceed and take quite a large part of the Dutch speaking population with it. Maybe the borders are just redrawn so the Netherlands includes most of Brabant, but not all of it (this Netherlands would lack Brussels). Result: you have the Netherlands, which includes most of Flemish speaking Belgium, and an independent Belgium, which has no hopes at all to over get sea access.