Try soaking the effected birds in warm Epsom salts water for about 20 minutes to loosen and remove feces and other debris around and in the vent area. Pat dry afterwards with paper towels or an old towel.
Let us know how it looks afterwards.
I suspect you're possibly dealing with scaly face mites. Treatment is Ivermectin Pour On. Dosage is 4 drops of Ivermectin Pour On on bare skin on the back of the neck. It will treat a 4-6lb hen. It will quickly be absorbed into the bloodstream and will kill the mites. Apply olive oil to the effected areas on the face, avoid the eyes. It will smother mites and soften the skin as well.
The swelling above the eye could be a peck wound or a scratch causing infection in or near the eye. Apply regular Neosporin or Terramycin eye ointment in the eye twice a day until healed.
You will also have to treat the coops. I recommend spraying inside coops with Permethrin.