Just wanted to give you some hope. I thought for sure my chickens has Coryza and Mg. So I submitted two chicken corpses for necropsy and cultures. Turns out they had neither. They did have some small issues, like worms and another bacteria. I am still waiting for the final results, but it seems it may have been a bad case of infectious bronchitis for the two that I euthanized.
Here is my thread that shows the reports, if you are interested.