Your ducks are babies and they're probably lonely. I always brooded my ducks inside until 8 weeks old. If it's hot, I would make sure they still have fresh water all night. Ducks are very sensitive to change and scary things, so you need to make sure they feel safe and secure. Don't get a drake...
How old are your girls? When I have brooded runners it seems like they go through a stage of trembling legs around 5-6 weeks old maybe. I do make sure they're getting plenty of niacin and add some vitamin B complex for a few weeks. Good advice above.
It would be suprising if all those breeds at the same time developed weak legs due to a niacin deficiency. Mazuri is a high quality food. Could it possibly be something else? Did it start around the time you moved them outside...even if they are afraid of things flying overhead, they might be...
Does she have access to daily fresh swimming water? That would be very important for ducks feather health! She will eventually molt and then can grow in better feathers.
Completely shell-less or do they have a paper thin shell or membrane? How is she doing today? Eating, active? If she has shown no improvement you should consider your vet euthanizing her. It might be best not to have her suffer :( Sounds like she may have a reproductive issue. It's so hard, but...