The stuff on the eye was pus. birds don't have it the same as we do, theirs is more cottage cheesy like. This was most likely caused by a respiratory disease, which one, I don't know without going through all of the symptoms.
It is possible that you transferred some type of illness to this bird, but I still say it was vitamin related. Either from bad feed or some type of a toxin playing havoc with the digestive system. I may be totally off the wall but from symptoms and such..... I would also say that these birds you got from the breeder are not the healthiest stock you could have taken in. I myself will not buy from a private person unless I have done my homework very well. Too many chances for problems this way. Many people out there insist that chickens get colds like we do. They think nothing of selling these birds or their young and never disclose the information to the buyer. Chickens don't get colds, they get diseases that do not go away and can be passed through the eggs to their young. These hatchlings may never show signs or symptoms of the disease but they can shed the virus they carry from their parent stock. Now if they were from a local breeder, chances are, they weren't vaccinated for Marek's either and in fact may have been exposed to it as chicks if it were anywhere in the breeder's flock. Could this have been Marek's, knowing they came from a breeder instead of a hatchery, yes it could have been. At this point, it really doesn't matter too much, except to use it as a warning to watch your remaining birds for any signs of illness.
The only Bantams I have are Showgirls, so I can't speak for the other varieties. I would say that the problems you are going through are, as said above, because of the breeding stock used. Some breeders are not careful of the lines of progression with their breeding. While chickens don't have a lot of issues with indiscriminate breeding practises at first, eventually you have to bring in stock from outside your flock. You can't keep breeding father- daughter, mother-son forever. Then there are brother-sister issues as well. If the gene pool isn't refreshed you will have weak and sickly stock that nothing but new blood is going to fix. If I had to guess, this is what happened to you and I truly am sorry you had to learn this the hard way.
It is possible that you transferred some type of illness to this bird, but I still say it was vitamin related. Either from bad feed or some type of a toxin playing havoc with the digestive system. I may be totally off the wall but from symptoms and such..... I would also say that these birds you got from the breeder are not the healthiest stock you could have taken in. I myself will not buy from a private person unless I have done my homework very well. Too many chances for problems this way. Many people out there insist that chickens get colds like we do. They think nothing of selling these birds or their young and never disclose the information to the buyer. Chickens don't get colds, they get diseases that do not go away and can be passed through the eggs to their young. These hatchlings may never show signs or symptoms of the disease but they can shed the virus they carry from their parent stock. Now if they were from a local breeder, chances are, they weren't vaccinated for Marek's either and in fact may have been exposed to it as chicks if it were anywhere in the breeder's flock. Could this have been Marek's, knowing they came from a breeder instead of a hatchery, yes it could have been. At this point, it really doesn't matter too much, except to use it as a warning to watch your remaining birds for any signs of illness.
The only Bantams I have are Showgirls, so I can't speak for the other varieties. I would say that the problems you are going through are, as said above, because of the breeding stock used. Some breeders are not careful of the lines of progression with their breeding. While chickens don't have a lot of issues with indiscriminate breeding practises at first, eventually you have to bring in stock from outside your flock. You can't keep breeding father- daughter, mother-son forever. Then there are brother-sister issues as well. If the gene pool isn't refreshed you will have weak and sickly stock that nothing but new blood is going to fix. If I had to guess, this is what happened to you and I truly am sorry you had to learn this the hard way.