So in the original thread there's a link to u/emdeyess's answer about the repeal of sodomy laws in France, and the reasons (and the answers to your detail questions) are essentially identical. This is because in 1794 France conquered the rough area now known as Belgium (then known as the Austrian Netherlands - the bit of the Netherlands that remained under Habsburg rule rather than being part of the Dutch Republic) to France, and thus French law applied.
'Belgium' didn't come into existence until the 1830s, at which point it had been a part of the French Republic, the French Empire, and the Kingdom of the Netherlands at various points between 1794 and 1831/2.