Hmmmm sounds like you shouldn't be having any problems. Mycoplasma does spread slowly though, and symptoms vary bird by bird but you are right if it's been months and no other chickens have signs of respiratory disease, that's good news. I'm sorry I don't have any other ideas what might be causing the swollen eyes and gasping.
The colour of hen's combs changes depending on where they are in their lay cycle- the rule of thumb is your fattest, best feathered, glossiest reddest combed hens are your lousiest layers wink Perhaps your setting hens aren't actually sick from anything? Are there any other symptoms?