Some questions off the top of my head although I think you've covered some of them...
Do they have plenty of room and no issues with them all being able to get to food and fresh water? Stress can cause a lowered immunitity which then could lead to infection being picked up.
As chookyinoz says, do they have deep enough water to put their whole face in? They do need this so they can clean out their nostrils and wash their eyes. Possibly they could develop an infection without being able to wash their face?
What is your water source? Is there a chance the water is high in cholorine or another chemical which could be affecting their health? I don't know what area you're from, has your water been affected by flooding at all?
Was the health of the breeding stock good at the time of collecting eggs? Good health is required for robust ducklings. Possibly you could try vitamins in their water as an added boost just in case?
Are the chicken crumbles fresh with no chance of mould? Two days into eating and then becoming sick might be as a result of food which isn't fresh? Have a really good look at the crumbles and if you're unsure at all, buy another lot from a different source.
Good luck finding out what the problem is, it would be very distressing to lose so many ducklings.