What are the consequences if William III, Prince of Orange and King of England is killed during his campaign in Ireland? How would Mary rule, what would happen in London, how would the king's campaigns be affected? Any ideas are welcome 
Consequences depend on the situation. Is he killed in the field at the Battle of the Boyne? The Williamites rout and James probably invades Scotland next, getting the highlanders riled up before marching south. Whether or not there will be a grand resistance to oppose him is hard to say given how badly he will have beaten them in Ireland
I wonder if Hendrik Casimir (the Frisian stadholder) would have enough cloud in the Netherlands to get himself elected as stadholder of the other provences and thus avoid the second stadholderless period. His son wasn't able to, possibly because he was too young and inexperienced (and didn't know the right people yet.
No, those troops have already the assurance that they will be paid at least for a period. There will also be no overdue payments. This reputation was the biggest strength of the Republic. So they will fight at least for a time. Morale would indeed be reduced, because of insecurity of the future.No, not in the battle itself but instead a day before. There is a convenient POD when he went on a reconnaissance and was shot in the shoulder but it was just a scratch. Let's say the projectile was fatal and shot him in the head maybe. The Williamites had numerical superiority, they were better trained and equipped but I've also read there were serious disagreements between his Dutch and English generals. Would the Dutch and their Danish and Huguenot mercenaries still fight a battle if William is dead and there are no Dutch interests in the war? The morale would also be reduced by the king's death too so that could change things a little.
The leadership falls apart basically.No, not in the battle itself but instead a day before. There is a convenient POD when he went on a reconnaissance and was shot in the shoulder but it was just a scratch. Let's say the projectile was fatal and shot him in the head maybe. The Williamites had numerical superiority, they were better trained and equipped but I've also read there were serious disagreements between his Dutch and English generals. Would the Dutch and their Danish and Huguenot mercenaries still fight a battle if William is dead and there are no Dutch interests in the war? The morale would also be reduced by the king's death too so that could change things a little.