UPDATE

soon for an update, I know - but this seems weird!)
I've had the sick girl inside confined in a small room under construction all day. A few hours ago I put some VetRx around her eyes and head (I'm doubtful, but it sounds like the oils might at least clear nasal passages and be antibacterial), and managed to get a little teroxy water in her. I came back a few hours later, and she had laid an egg in the corner!
She was still listless, and I tried to give her more water and no luck force feeding (I didn't want to choke her) except a tiny piece of tomato. I held her on my lap for a long time. Then she kind of suddenly got up and started walking around, eyes wide open. She drank and ate a little on her own, then she started pacing and wanting outside where she could hear the other chickens. I let her go out, and she made a beeline for the coop, had some more water and went to her nesting box. After a while she came back out (no egg), and has been hanging out with the others - not all there, but up and eyes mostly open.
Now another hen has slightly puffy eyes and a runny nose, but her usual appetite, and the other two look normal.
Should I watch and wait? Am I wrong to let her back with the others?