I hear ideal is a very reputable hatchery, so don't blame them to quickly. Sometimes babies just don't survive shipping. Don't blame yourself either, you're going everything in your power.
Keep us updated. :)
You'll also need to go to the pharmacy and get plain Niacin tablets (tell the pharmacist NOT time released or flush free, just the normal plain Niacin). It's Vitamin B that ducklings require more than chicks. Crush them up and put 100 mg per gallon in their water.
Or you can sprinkle brewers...
You're doing everything I would know to do.
Electrolytes
Wet food
The only other thing I would do is quarantine and that's only to protect the others... and maybe some probiotic in their feed.
Prayers for baby Cayuga