My duck is mounting my hen....
There's really nothing you can do to stop it unless you separate him from the girls. It's normal for them to pull out feather, they bite them to first get the female to lay down then to hold on while they do their thing. She wants to hatch babies so she's letting him breed her. Unless there's carnage you shouldn't worry. Unless you don't want ducklings, just take eggs the day they're laid and put the them in the fridge and they won't hatch. If one goes broody you can take the eggs from under her and get her off the nest.
If your wife isn't too attached to them but still is set on having ducks, you may be able to compromise. You could get rid of the drakes and order two sexed female ducklings from a hatchery online, like Metzer or Meyer. There are lots of hatcheries that sell sexed chicks via mail. If you don't havery the heart to butcher, you can try re homing them, or even try your local shelter. One of the shelters near here will take any kind of farm animals. If she doesn't want to get rid of them, the best option would be to somehow separate them. You could also try adding a few female ducks to your flock and a kiddie pool. Drakes much prefer to mate on water, so if he sees females smashing around, it may take his attention from the hens a bit.
I figured I would ask for some advice in hopes that there would be some what of an easier way. Thank you again.