UGH! I recently had a fecal done that came back negative and saw a broody poo out a large round worm. It's unfortunate that the test isn't always accurate. But sometimes it takes a little while for things to show up. I have recently heard retesting in 2 weeks could give dramatically different results. I'll be honest testing gets expensive. In addition, some birds have better immunity fighting things off. The samole from just two birds might not give a broad enough view. It's awesome that you have already sought vet help...
How long has it been since your sneezing? Only about 10 days... And where did your birds come from? Black sex link, I thought would be from hatchery which shouldn't have any disease issues... but hard to say what could have been at your property already... Or if you got them private party. I do not know the time frame it takes for that stuff to be considered clean from your ground.
My best guess about PNW... in this instance might be Purina nature wise feed? Please tell us the protein and calcium %?
Antibiotics can be hard on the system... And cause diarrhea... your conditions do sound right for cocci... but with your other concerns of sneezing and such could be a perfect storm of something trying to take her down. I agree that hydration is important and good for you for doing something! Now I think I would return her to the flock, since you haven't seen any actual bullying. And offer some sort of probiotic, either some plain yogurt, a probiotoc and electrolyte mix, or some raw acv added to drinking water... get the gut flora back on track to support natural immunity antibiotics kill both beneficial and harmful bacteria.
IF I believed it was cocci, I would not supplement vitamins at the same time as treatment. The corid works by blocking thiamine to starve out the cocci and slow their growth while the birds build their own resistance.
