Erin, one reason to separate the sick hen is to keep a close eye on manure output (none you know you have a problem!) and laying. I'd suspect the hen that doesn't eat. That's always a sign. Without a lab test it's about impossible to know what bacterial infection you have. Any broad spectrum antibiotic would do at this point. As far as which came first the infection or the stuck egg, it's probably a bit of both. All chickens some some levels of bacteria in their systems. When something goes awry, infection occurs. It's likely that for some reason the laying "conveyor belt" slowed or stopped. (possibly due to disease, or stress or mechanical failure) Infection and impaction occurred.
Since your hen passed that mass, I'd say there's cause for optimism.
Since your hen passed that mass, I'd say there's cause for optimism.