I would begin B vitamin supplementation, perhaps make a small hammock so that the duckling can sit upright, with an opening so that the poop can drop to the floor and not build up against the duckling's body.
Gentle massage may help.
We cannot know how much of a recovery they can make, but generally ducklings can do much better than we imagine - as long as they get the best support, nutritionally, and in their environment.
If you can get B Complex vitamins, and add that to the drinking water, that's a good start. I would use the amount of niacin - B3 - as a guide. 150 mg niacin per gallon of water.
Can you open the toes and lay the foot flat with our any trouble if you can then make the shoes others have for their ducklings and they have worked. and def get the duckling on B complex like Amiga says.