I'm so sorry that you have to go through this. Here are some good articles on MG that may answer some of your questions:
http://edis.ifas.ufl.edu/ps034
http://umaine.edu/livestock/poultry/mycoplasma-gallisepticum-faq/...
Any chickens that have been around them could possibly be carriers of MG, even if they do not show symptoms. You could contact your state vet about getting others tested. As long as there is an MG positive bird left alive, you could have the disease come up again to any new birds. MG only stays...
Sorry about all of the illness, ans having to cull your rooster. Hopefully no one else gets sick. Thanks for updating your thread--it really is good for others to read of the risk of buying birds that can carry diseases into their flocks. Quarantining new birds for 30-45 days is a good practice.
Have you added any new chickens to the flock lately? MG and coryza are some of the respiratory diseases that can cause swollen pus-filled eyes. The eyes can also be infected with other bacteria also. MG and coryza can make carriers of any chickens exposed, for life. Does he have a bad odor...
You can put some iodine on your finger and dab it on the peck mark. BluKote would work also, but don't get it near his eye (caution it is very staining, so use a rubber glove to dab it on.) BluKote comes in dab-on or spray. Plain old gentian violet in a small bottle from WalMart first aid...
He has a mark on the swollen area, so I would suspect a peck wound if he is not showing signs of a respiratory infection. Symptoms would include bubbles or sticky drainage in the eye, nasal secretions, cough, sneeze, or rattly breathing. Watch him to see if the swelling starts to go down, or...