Ah you poor thing. I know the feeling, being a first time chicken owner is hard. They are such fragile little creature and you really do get attached. People make out theyre such easy pets but really its not that simple.
I havent personally dealt with coyza so I cant be sure but I dont particularly like the sound of the smelly mucus. However, when you read online it seems it can always be 10 different things when it comes to hens so its hard to tell. But its good that the antibiotics seem to be easing it up. I do however know that MG does cause bubbly eyes, due to it being a respiratory disease. And if it is that then to answer your question, if she has been with the flock for this long than they have more than likely already contracted it and are now all carriers. Don't fret, if it is MG, if its treated than it can be managed and often isnt fatal. MG symptoms (like bubbly eyes and sneezing) seem to spring up when a hen is stressed and that can happen so easily. Chickens stress over the smallest things, this can be from change in weather or feed to change in their environment completely. But as soon as they do show these symptoms, put the antibiotics in all of their water. And if its just one hen looking sickly, isolate it and keep it warm. Isolating may not always be ideal for the owner of the hen but it is good so that the hen wont affect the other hens but also so she has a chance to recover in peace and so she doesnt get bullied by the flock for being weak. As for how she contracted it, that could be a number of things. Things like this are often in the air if I remember rightly, she could have well likely already had it when you bought her (unknown to the sellers) and is only just showing signs now. Hens can catch diseases from, being airborn diseases, other chickens with the illness, even dander on the breeze, mosquitoes, other wild birds, from the ground. I could go on. Some people have no troubles, others have their hens constantly ill. Unfortunately I am one with all the troubles. I know its hard when they are like this, especially if they are your little pets but you can only do so much unfortunately.
I will try and link you to a post I put up about my first hen who had MG and you may find some more info from there until you get ore replies on this thread. (Here it is:
https://www.backyardchickens.com/forum/newestpost/1022565 ) but keep looking stuff up on more websites.
Im sorry you are in this situation and I do hope your girl recovers quickly. Try not to stress too much x I know every time I have a hen problem, I come on here and whinge like a baby. I really care for my hens and know how hard it is to see them ill but the people on here are really supportive and so amazing and really know their stuff. Also a word of advice. Apple cider vinegar is great stuff for hens, just put a few drops in their water. Many people swear by it to help with everything chicken, its just good for them in general so i always like to keep some around and add a few drops into the water. Others like to spray hens with water mixed with a few drops of ACV when its hot and its said to help keep parasites away but I wont swear by that. And plain sugar free yogurt is also good for the immune system. There are plenty of helpful herbs for hens!