Ideally, you could quickly get them to a good duck vet - just want to write that on the thread for anyone reading this in the future.
Could be some kind of toxicity, yes - I would change the food - you are trying some good things, like the electrolytes.
I would bring them into a pen with...
Doesn't sound like a broken neck - sounds like something internal. I would consider an oral or injectable antibiotic. A vet would be best, and I understand not always manageable.
If it has been a few days, a flush may not be much help - the activated charcoal, definitely.
Also could be advanced infection. In which case antibiotics needed.