Hi! Welcome.
I will try to answer your questions.
I would probably recommend against doing this for several reasons. One of them is that it's not a good idea to mix wild and feral ducks with your own ducks. Even if your ducks appear healthy, they can spread disease to the wild ducks and vice versa. Wild and feral ducks tend to have a lot of parasites as well.
I've heard a lot of stories about ducks not returning for night-time lockup and staying out on the lake/ponds refusing to come in. They can be trained to come back, but don't always want do so.
Yes, they will breed with the wild ducks diluting the wild duck gene pool. This may result in a larger number of non-flying ducks that can't migrate. There's not much you can do about them breeding as it's a strong drive for them to do so. You can take away any eggs your ducks lay (if they don't hide them from you), but you can't mess with a wild duck's nest.
Also, I don't know if your lake is a man-made lake or a lake maintained by a Home Owner's Association. If it is, they probably wouldn't like you adding to what might be an existing problem.
Free-ranging, in general, is risky even if it's only done for a few hours a day. You will lose birds that way, guaranteed.