Wow, that was a good post.
I really feel for you because I am the same way and I try to save all of them too. 
Sometimes Corid will work, sometimes it won't. It's so hard to know what's making them sick, so we do the best that we can with whatever we can get our hands on.
Keeping her inside with the others is fine as long as they don't trample her, and treating them with Corid is safe even if they aren't sick.
I really feel for you because I am the same way and I try to save all of them too. 
Sometimes Corid will work, sometimes it won't. It's so hard to know what's making them sick, so we do the best that we can with whatever we can get our hands on.
Keeping her inside with the others is fine as long as they don't trample her, and treating them with Corid is safe even if they aren't sick.
. Thank you for putting into words what I have always felt and have a hard time explaining to my husband. "They're chickens" is his answer to most of my fussing. It's so difficult to see them suffering, and I'm sending healing energy to your chick and hugs from Michigan to you.