I had extra basil this year. They ate that too. They seem to prefer softer stemmed herbs such as basil or parsley rather than harder stems like thyme or oregano. Next year I may plant more herbs- just for the ducks.
My ducks eat wilted lettuce, bruised fruit, assorted scraps such as the seeds inside bell peppers. Sometimes when I've offered new things, they are unenthused but usually go back and try it later. A winter treat they really like are baked potatos.
I've had this happen too. There's no mark on the foot, no bone sticking out of the skin... There's not much you can do. Most likely, your duck will limp for a while then recover on his own.
You better keep them separated. It sounds like the two new ducks are a team and are not open to any other ducks joining them. You could add more ducks. You'll have to watch and be sure nobody is being picked on.
Personally, I like duck eggs better than chicken eggs. They have more flavor and are more substantial in texture- whether it's in a cake, scrambled or whatever. Hard boiled duck eggs are wonderful!
Surely, some of the folks at church will at least be willing to try some duck eggs for free.
I have a runner with a poof of feathers on his head. He had it when he was a day old. (Tried to upload pictures but kept getting an error message) He's three years old now, still has the poof. He's a happy drake watching over three campbell gals.