Here is the list of state vets and the link for Zoologix:
https://www.metzerfarms.com/poultry-labs.html
https://www.zoologix.com/
https://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
Vet Rx is herbal oils plus camphor, and does not treat anything. It is similar to using Vicks. Besides MG and infectious bronchitis, there is ILT, coryza, and others. You might think about getting some testing through your state vet lab or Zoologix, a national lab.
Has she ever had a swollen eyelid or face, or bubbles/foam in one eye? Viruses can last around a month, and antibiotics do not treat those.Mg can be treated with some antibiotics, but it always remains in the body as a carrier. Symptoms may go away.