The triple whammy should be pretty effective.
Especially since that combination would also cause quite significant long term economic problems. Probably the VoC goes bankrupt, and the whole Dutch East India empire falls apart.
But an invasion might have some issues. Firstly, England is (was OTL anyway) pretty occupied herself, what with the Great Plague, followed by the Great Fire. And, if there is a really bad outbreak of plague in the Netherlands, is anyone really going to want to invade ? Most people would be trying to get away, not the reverse. However, if 'invasion' means seizing the key Dutch fortresses , then that should be very doable.
The Spanish might very possibly want to send troops, to enforce a cordon sanitaire, to keep infected people escaping out of Netherlands into Flanders. And the Bishop of Munster has already invaded (though I've never been clear why - maybe he just liked invading places) . I really think that after that triple whammy (especially if the 1666 re-outbreak is really severe), the Dutch won't need any encouragement to the negotiating table. They're not in a position to argue,.
As to William as Stadtholder, that seems quite feasible. In times of national disaster , people aren't all that rational. They'll likely blame "the government" (ie de Witt) for not doing enough - or doing too much . And religious tempers being what they were, someone will probably declare that it is all the Judgement of God against a sinful people. Obviously, God hates the present government. William, as the alternative , should be a shoe-in. Just one point, though, William in 1666 is only 16 years old.
If the plague was bad, really bad, which it could well be, William's guardians might well want to send him away somewhere safe- like England, where his guardian , King Charles, lives, maybe ?
What are the war aims, though, and the possible alliances.
England wants to cripple Dutch mercantile power. Done.
Charles wants some glorious victories, to make people happy. Done. And an excuse to build up an army.
Spain wants the Dutch crushed (and probably, if possible, re-annexed).
France wants Flanders.
At this stage, there is no legal restraint on Charles having as big an army as he can afford. Afford being the operative word. If Parliament won't give him money, maybe either Spain or France will, in exchange for his alliance? He could probably go in on either side, neither country was popular with the English people.
Extreme case, the Dutch Republic could end up dismembered, and partitioned out. England had always wanted a replacement for Calais- hence why Charles selling Dunkirk was so very unpopular. Maybe now, he takes it back ? In exchange, Louis takes Flanders (not Antwerp, though)