Yellow matter she sneezed looks like the egg you gave her,but could also be bile or mucus,i feel she has a severe respiratory infection. You mentioned that she has been congested for quite some time,i think it is time to switch to a stronger antibiotic. She appears to be having trouble breathing,keep an eye on her crop this is also a concern. She needs veterinary attention.
I personally have never used anything but Tetracycline,she probably needs Tylan(Tylosin). PM Casportpony for Tylan dosage information. Listen to her chest,can you hear any wheezing from lungs?
If no other medications are available,then keep using the Duramycin. Keep her warm,use subdued lighting,if she still show signs of difficulty breathing try a humidifier. I am guessing that whatever she has is bacterial,if it was a virus your other chickens would be ill. Watch her crop,as sour crop has not been ruled out. Have you opened her beak to look for anything unusual?
I only know of Tylan(Tylosin) or Baytril(may need a vet prescription for Baytril,but ask anyway).
