Duck waste will break down quickly. My ducks are currently on dirt/mud and there is no build up. Pea gravel has been recommended by a few duck breeders of note. So I beleive it would be good. It is on my to-do list. I have not implmented it yet. But you could add a drain away underneath the...
Yes. More drakes will get along. When a female enters the picture, the drakes lose their minds. They get competitive. If you are just looking for pets, all drakes is a great way to go.
Pea gravel. You can hose it off and the waste will just sink below it. Pavers or concrete of any sort will be hard on webbed feet. It will increase the possibility of getting bumblefoot (not assured - but increase it). If you dig out an area and add a drain pipe underneath it will be even...
I put niacin from vitamins (crushed) into water in a coffee can seperate from the other water so they can self regulate. It seems to work. FWIW. Yes, they waste some but the bottle has lasted for years. :)
As far as the drakes behavior to each other, They are establishing pecking order. They will work things out and settle down. And every once in a while they will go at it again.
If they go after you. Grab them and sit on them for 2, better yet 3 or 4 minutes (use a timer, 30 seconds wont work)...
Snappers eat ducks. But my understanding is that they usually come up from underneath and eat the young ones on ponds. But if one developed a taste - they might pursue them. They are diggers and can get big and strong.
Easily could be a wild female sneaking in for a safe place to nest. The males may have attracted her. Leave the eggs, you might wind up with a free replacement female. If you keep picking them up, she will decide that this not a good place to nest and find another spot.
Racoons are WAY WAY stronger than you might realize. They are like miniature bears. Also, when you said a hole was dug and rocks moved I immediately thought snapping turtle. Don't know your environment or if any of those might be around.