The sores on the feet look like bumblefoot to me--a bacterial infection (staphylococcus) that gets in through cuts or scrapes on the bottoms of the feet. It's very common--staph bacteria is everywhere! There are topical treatments (both sprays and ointments) that can help a lot. Often, just getting the birds out of any damp environment and standing on nice, dry, smooth ground for a week or so will allow their immune system to clear it up naturally. Occasionally an abscess might form, and then you can lance it, cover it with an anti-bacterial salve like Neosporin, and wrap the foot with gauze and VetWrap. You'll want to keep that animal in a confined, dry space while healing, too, of course. If you need to confine ducks away from access to ponds or tubs of water, keep a close eye on their nostrils, eyes, and uropygial gland (the oil gland on top of their tail, near the base of the spine) for signs of inflammation or irritation. If they are getting irritated or if the gland seems blocked, remove the foot bandage and give them supervised time with a shallow tub of water so they can preen and get their skin and nostrils back to condition. After they've had a bath, dry the foot thoroughly, re-apply the salve, and re-bandage.
Your mention of birds keeling over dead overnight, and birds with stiffness, also has me suspicious of botulinum toxin. This comes from eating something that has spoiled, and has grown clostridium bacteria on the surface. All birds (and humans) are susceptible to it--I've even seen a dog paralyzed from botox poisoning--and yes, it's the same botox some people inject into their faces to prevent wrinkles! But waterfowl, particularly ducks, seem to have extreme susceptibility to botulism poisoning and paralysis. So be sure you're inspecting their food and their enclosures carefully for signs of spoilage. Mid-rotation, walk their enclosures daily and pick up any dead rodents or songbirds you find (a prime source of botulism), any really old fruit that may have fallen off trees, etc.