I'm starting to think it might be chlamydiosis. She looks exactly like the duck pictured in this thread. The neurological symptoms are the same, however no nasal discharge or respiratory symptoms.
https://www.backyardchickens.com/t/368921/duckling-with-twisting-neck-very-concerned-please-help/10
For all I know, she got it from her previous home. That guy could have parrots too? It looks like it typically rears its ugly head at around 4 weeks, which is precisely her age when the symptoms began.
I don't think it's botulism, because #1 I don't see how she could get it? At least not here on my property. #2 her neck is stiff, not flacid and her legs are not paralyzed, she just can't walk.
I'm going to give tetra or doxycycline a try while continuing with the nutritional yeast, and I'll put polyvisol in her water. I will most likely have to assist her in eating and drinking at this point. If this doesn't work, I'll have to cull.
I guess I learned my lesson. Don't buy birds from un-known breeders.