Thanks for the replies. I know there isn't much to go on here.
Windy - I think you are completely correct in every regard about the banty chick we lost. In that case, the severe coccid infection was diagnosed with a fecal, and death appeared to be due to internal bleeding. Her feet were cold...
I'm afraid this is going to sound like many other long, hopeless posts, and there may be little useful advice you can offer, but I'm desperate.
My wife and I started a small urban chicken flock in Seattle in April. We got 3 chicks from a local feed store, and all of them grew into healthy...