Why do you think that? Willem's proposal on 26 august 1650 for mediation between France and Spain, that heavily favored France, and that had to lead the Republic in a war with Spain, was consequently neutered by the representatives of Holland. It showed Willem's earlier victory was Pyrrhic
His uncle had also replaced all opposing regents. And within a few years the ballance of power had shifted back to the old situation.
The cities in Zeeland did care, they get direct income from blockading Antwerp.
Only 4 Hollandse cities supported Willem II openly in the crisis of 1649/1650 leading to the strike on Amsterdam. Willem II was lucky to get the favorable outcome he got.
Another reason for the existing states and cities to stop their bickering and unite against this idea, that would limit the influence of all. The formal argument used would be that the government of the Republic was already difficult enough, because decisions could be only made in total unity.
So Willem might not have the finances/troops to go another round against the Regents? Still, it's hardly as though Spain was in better straits financially IIRC. Willem also doesn't seem like the type that would let lacking finances/troops stop him. But I
do agree that Wim is likely to sue for a peace soon - but he'll want peace on his terms. Whether he gets that, is another matter entirely.
As to replacing the former regents, it usually goes like that with a packed deck - namely you can pack the house with cronies, but sooner or later the cronies are no longer gonna be yes-men or your yes-men are going to start dying off/the opposition is going to get a strong player.
As to the Southern Netherlands - couldn't Wim just directly annex it as personal holdings? That way he'd get a power base, the states would be directly beholden to him rather than a (entirely?) Protestant Staaten-Generaal.
If William II succeeds in taking a part of the Spanish Netherlands what would be there status?
IMO Holland saw France as a good friend but bad neighbour there are few or none benefits from any partition of Southern Netherlands between republic and France
France was seen as a bad neighbour during Louis XIV's constant wars in the Southern Netherlands. I don't say that
those won't happen here -
but (using OTL as an example), his justification for several wars that he fought in the Southern Netherlands were wars against the larger Spanish Empire, in the name of his wife - for whom he claimed them as inheritance (
jus devolutionis) when her dad died or in lieu of an unpaid dowry. If they no longer belong to Spain, but to the Netherlands, then Louis' whole raison de guerre disappears. I don't say this will stop him attacking - he'll find a reason, since as Friedrich the Great said to a minister when looking for a pretext to march into Silesia in 1740 "that's the work of a good charlatan" - but he
won't be attacking Spanish territories, and can't use his OTL reasons. Rather, he'd be turning on an erstwhile ally, which is going to look considerably worse for France in the international arena (it's going to make the king look as though he can't be trusted to respect anyone. And the Dutch will go shopping for allies elsewhere as OTL).
As to Cromwell allying with Spain, it's an interesting idea. However, Spain might demand their fellow-believers in Ireland be left alone - and I don't know if Ollie's gonna play along there. Felipe II might have been pragmatic enough to let alone, Olivares too. Felipe IV from all I've read on him (wasn't Haro advising him at this point? or was it personal rule again? I remember he said after the
valido was dismissed "I am quite old enough to see things clearly for myself" - and then carried on as before. But I can't remember who was in charge in the early 1650s) doesn't strike me as the pragmatic sort - unless it comes to finding a dress that's easier for him to get into/a mistress out of.