@Miss Lydia Has rats around her coop I believe. I wouldn't leave the duck food out at night, I doubt the ducks are eating it at night your just attracting the critters.
I would be concerned about the long term effects of the ducklings drinking chlorinated water on a regular basis, not to mention chlorine can irritate the eyes and skin of animals. I think it would be best not to use chlorine in the pool at all. IMO.
The pool doesn't look that bad, come over to my pond in dead summer and you'll think yours is clean mountain water. Definitely don't put any chlorine in there,
Hmm, Lots of animals will take ducklings, so its hard to say. I see Hawks, Mink, Foxes and sometimes Owls during the day, but if its hungry it will take them anytime it has the chance.
Hard to tell, could be owl, mink, fox, really any nocturnal predator that is a common occurrence in your area. You'd be amazed how small of holes minks can fit through and how high foxes can jump. Are the babies missing or are their signs of the attack?
Very sorry for your loss! :hugs