I just saw this thread and read through. Lots of good advice from everyone. I think you have a rat. You had openings big enough for it to fit in. You have straw that it can hide in. There are no tracks because they burrow under and you wouldn't see tracks. They can get in during the day when the coop is open and hide out until the birds are in to roost and will attack when they are unable to to run away. They attack from below, which would explain why the bottoms were attacked. A weasel will attack the neck, drink out the blood and leave the rest. I know this for fact because that is exactly what one did to 4 of my rabbits, and that is how you identify a weasel attack. Rats and mice will come around where ever you have grains. I would set a small animals trap, like one for squirrels and chipmunks, inside the coop with some bait (not poison). The chickens can't get into it, but the rodent can. Even if you have sealed up everything, as long as it is open at all, a rodent could get in.
Personally, I don't think that your rogue hen did the original attack. I think once there was blood, she joined in the gruesome feast. She perhaps was in a frenzy because of it, or just thought that the roo had a bloody spot to peck off. Anyway, it is probably good you got her out of there.
I hope this is helpful and you catch the predator you have lurking about.