Ok..  the mites are drastically reduced.  Spur feeds didn't have anything to help with the mites.. They wanted me to use a sulphur ointment.. on feathers.  SO,  I used the sevin dust and the mites all ran to his head..  then he tucked his head under his wing to sleep and got sevin on his head.   He still has mites but he is quickly improving on that front.  I also used the DE mixed into his pine bedding and the sevin dust under his bedding.
He LOVES the chicken layer pellets, drinks constantly, and poops a LOT of soupy greenish poop. I am having to clean the bedding in his box twice a day.  His eyes are bright and shiny, and he flicks his tail in what I think is happiness...  he also talks to me when I enter the room.  His quiet noises are sooo sweet. 
Now the (broken?) leg.  He isn't walking on it at all.  The first morning after I rescued him, he hobbled out of his box and chased my chihuahua away from his dog food and started stealing it.  But since then he hasn't left the box, of course I have tried to give him as little need to leave the box as possible and have it set up so his water is outside the box but up against it  and his food is in a dish in his box.   it's his left leg, and it's swollen right at the joint.  He can't flex it, and he keeps it slightly extended behind him, which forces him to lean to the right.  It's the location of the swelling that has me doubting it's a broken leg.
When I had my starling, I learned that a diluted topical application of DMSO could be used on some birds to relieve pain and swelling.  Are ducks part of that group?   I know he must hurt because he's in the warmest room of my house, sitting on soft pine bedding with both food and water within necks reach - and he suddenly starts shivering.  
I've asked around a bit at work about local vets who treat ducks... or chickens... and no luck.  The couple of vets I've called are all strictly mammal vets - so I am all he has.