Peanut butter color poop?

[Eggcessive]

It sounds like you need to see a vet, or contact your state vet to get a necropsy. You have a lot going on, possibly one or more diseases. Viruses like ILT will not respond to antibiotics. Coryza or a secondary infection of E.coli will probably not respond to Tylan or Denagard. Coryza is commonly treated with sulfadimethoxine, sometimes in combination with Tylan. We are not vets here, so you may need to get testing or one sent in for a necropsy. It is sad to have this much going on, but you need to try and find out what it is.

 

[Wyorp Rock]

I have given each of those hens oral Tylan 50 and put Denagard in the waterers to treat everyone.

And while they all have the one-eyed swelling, a couple of the hens end up with swelling on both sides, along with swollen wattles and throat. So swollen they look like they could pop.
There is yellowish / greenish diarrhea in the coop

Nobody has a terrible smell, but it seems that that is not necessary a requirement of Coryza.

It was very cold and wet and then very warm here recently, and that's when all this hit. The coop suddenly had an ammonia spike, so I cleaned with Oxine, removed anything damp, put in fresh shavings. I did do the big-time no no of buying 2 five-month-old birds from someone. I did bring them in my house and away from my flock at first, and I did discover that they were sick- they had some nasal discharge and sneezing. I had treated them with Tylan, and they recovered nicely. After 10 days, I introduced them into my coop. I am fairly certain that this was my root cause, but I don't know why my birds seem SO much sicker!

Has this just become survival of the fittest? If one dies, does anyone know how I can find out what this illness was in Virginia?

I'm very sorry that your birds are all getting sick.
Since you are not really seeing improvement with the Tylan50 and/or Denagard, I agree it's time to seek professional help and find out what's going on.
I found the link to your state lab, that would be the place to start. They should have the latest information about what diseases are prevalent in your area and can assist you with testing.
http://www.vdacs.virginia.gov/animals-animal-health-laboratory-services.shtml