I'm sorry you're having such troubles with your Nankins.
The way to find out what the vet lab needs for a proper necropsy is to call them and ask (explain what you suspect the problem is, then ask if it matters whether the bird dies naturally or is euthanized, and if euthanasia is ok then is there a particular method you need to use or avoid). Don't google, just call them, they can tell you CORRECTLY.
I am not grasping why you cannot just drop the breeder a line and say you think this problem is occurring, without needing documentation and paperwork and all. I can't help but feel there is a LOT of other story here we're not getting. It really does not need to be a big "thing"...
Good luck,
Pat