Other than one who'd gone broody (so I think it was linked to stress/nutrition), I've never had a pullet go through a molt their first year, so I doubt it's molting.
Did you look down to skin level and at the base of the feather shafts - around the vent area, under the wings, and even around the neck - for lice or mites??? Lice are fairly easy to see. I think mites are harder, but you'll see 'dirty looking stuff' at the base of the feathers.
You might wait to isolate the bully until you put the blue kote on so you can see who's beak might be blue. And since it's the weekend, you can spend some time observing them.
You probably have a bully, but tight spaces seem to exasperate the problem - do your birds have ample space? Usually there's a main bully in bullying cases, so you wouldn't necessarily have to isolate the "followers." Isolating just one might set up a totally different dynamic in your flock.
Unless there's an open wound on any of the feather plucked birds, you shouldn't need to isolate them as long as you coated the area well. If it comes down to it, chicken saddles may help. And if you get desperate, look into pinless peepers (kind of like horse blinders for birds).