My advice is to never go back to that place.
They actually said dump him off at a park well that is completely against the law and these people should know a domestic duck that has been cared for by humans has very little chance of making it out in the wild...
Does Sparki mind the grocery bag diaper? If he doesn't and it's not causing him problems I would continue to use them.
To me it sounds like ya'll love Sparki and only want the best for him.
You don’t want to put grit in their food just a small bowl close to their food and let them eat it as they need it.
It looks to me like he may have a niacin deficiency I’d get liquid BComplex and start giving him 1 ml daily and poultry cell to boost his immune system. As for feed my Muscovy...