there isnt anything you can do to improve chances of survival of released mallards, even if the mallard accepts outdoors, ponds and lakes, the predator environment has grown dramatically. in my location on a lake we had hundreds of ducklings survived every year to adult with their moms. but over the past 10 years predators killed almost all of them. what has happened is tremendous resurgence of hawks and their kills have made foxes and coyotes much bolder in finding animals and birds to eat.
the hawk can move quickly and kill very large birds even in the water by drowning them, and the eagle as the hawk is on top of the food chain. those ducklings the eagle killed wouldnt have had a chance against any predator bird. they were doomed as are all wild ducks today unable to deal with increasing hostile environments. even seagulls pick off ducklings, if ducks see the dangers from the sky they do attempt to escape diving under water but its only a matter of time.. nights when ducks rest they must lie on the ground and that is when foxes finish them off.
if you have rescued a mallard or wood duck, it wouldnt have survived anyway, mother mallards are in extreme danger also and this is why ducklings are found in backyards, parking lots and wandering alone... as far as laws prohibiting release of reared ducklings? the laws never protected USA birds when it came to introduction of diseased flamingos from africa... and this was a prominent USA zoo believed responsible for millions of bird deaths.
how dangerous is life in the wild? a cardinals life span is about 1 year outdoors.. in captivity a cardinal lived to 26 years.