Just an update, in case anyone has a similar situation in the future.
2 days after my post, the Big Chicken did in fact lay her first egg. Things have been pretty calm and pleasant since then. There obviously is some normal chicken aggression, but nothing like before. They will actually all sit outside together pecking around in the dirt, and happily roost on their perch together. Maybe it had to do with chicken PMS, or maybe it was just a matter of time, but everything is just fine now. So there is always hope!
And on a totally unrelated note, Big Chicken appears to be laying 2 eggs at a time... Yes, I know that is not supposed to be possible, but after the 4th pair of eggs popped out, I am beginning to have my suspicions as to whether Big Chicken defies the laws of Chicken Physiology. I had just been in the coop 2 minutes before, and when I came back in, there were 2 warm eggs in the middle of a nice nest she had made. Every other day 2 eggs pop out, and I'm pretty sure it is not the 3 month olds laying eggs... But, I digress.