Keep an eye on your birds that were in the same coop with the sick birds for one month.
Surely you'll see MG symptoms in about a weeks time frame. You can put stress on them; after 1 week, chase them around in the pen with a broom a few times on different days. That'll get them stressed for sure...
You're in a difficult situation for sure. I would not sell nor give away any birds whatsoever, chicks included.
For Coryza, symptoms appear in 3 days and can easily and quickly spread through a flock. You should already know the symptoms, see the link below.
For MG, the incubation period is 6...