What if the United Dutch states managed to remain independent, and how could this be accomplished?
First you need to beat the Austrians. For that you need some international support. The Dutch republic migth support them, but that alone is not enough. France might support them, but you don´t want that. The only reason for French support is to annex the southern Netherlands later. I think you need the British or the Prussians for it to work. Also you need to give Austria a reason to let them go, perhaps they are busy fighting someone else. No idea who would be good candidates at that time. France maybe? The Ottomans? Local revolts?
Next thing is to keep the French out. This part is hard, as the French have tried to annex the southern Netherlands since the 16th century (or possibly even earlier). I think the best option for this is a failed French revolution. That would cause chaos in all of France with republican and royalist forces fighting each other, so they can't look north for easy enlargements. This mighth weaken France enough that they won't try to annex this new Belgium the moment they are stable again. Maybe this is the cause for the Austrian distraction, trying to restore the French king.
Another important thing will be to keep the Dutch out. This is relatively easy as the Dutch usually had not enough interest in the southern netherlands to fight a war against a major power over it, but a small independent Belgium kingdom could be enough reason to annex it. But I doubt that. Also many of the revolutionaries wanted the southern Netherlands to become part of the Dutch republic. You have to prevent that happening. I can easily see the Dutch accepting them. I think the best way to prevent this is that the southern Netherlands were catholic and catholics weren't equal citizens in the netherlands. There were enough people who wanted to change that (like the patriotten), but they still weren't equal. This might be enough reason for the southerners to not wanting to become part of the republic or enough reason for the Dutch to refuse (why add a lot of new catholics to a protestant country, this can only lead to trouble). But I think you need republican Dutch support for "the United Dutch states" to exist, maybe some kind of early benelux forms. Two countries cooperating in a lot of ways, but still two independent countries.