I wouldn't release them unless you are willing to feed them too, they are domesticated and will be dependent on you for food, at least till the ones that have migrated have come back in the spring then they will probably find their own mates and become wild. You sound like a responsible person...
Might be your best bet unless you can find more females. but then you still don't have any guarantee the drakes still won't fight, I had 2 drakes to 10 girls till this past summer when i decided to keep another drake from a hatch ,and those boys still would fight and get bloody. No telling what...
I don't want to alarm you any more than you already are and my experience is with Muscovy's only, but back when i first started with ducks and did not know beans about them, they just appeared in the river below our home, 3 drakes. found out later about dumping, I decided to adopt them oh they...