That still requires a Franco-Belgian border commission.
I haven’t seen the treaties and laws defining the border, but the reactions from both countries suggest that the border is defined by the position of the marker stone. The people who made that definition are long dead, and there wasn’t a problem for years after.
As for my solution, I don’t think abolishing all nations and borders will be seen as an acceptable option, unfortunately.
A little known fact is that at many places nobody knows where the Belgian borders are anyway. They were crudely defined in the Treaty of London and afterwards nobody could be bothered to work out the kinks. They mostly follow ancient feudal boundaries that even local historians forgot.
There is even quite a bit of chunk of land at Vaals where Belgium, Germany and The Netherlands border(*) that nobody seems to claim. The Belgians and Dutch are not going to risk a good and fruitful cooperation over a bit of dirt and about the Germans and the Belgian border, least said…
(*Apropos of nothing but… The tourist trap there called Drielandenpunt is arguably the place least worth visiting there within 200km )
More than you think. My Borderer ancestors didn’t respect the border between England and Scotland. It took centuries and the unification of the crowns to control us.