It sounds like you have spent a lot of time on your plans! I have never had a broody hatch chicks before so this question might sound silly, but if you brood in an elevated coop, can the chicks get down the ramp okay?
I admire your concern for chickeners faced with evacuation. Every year we hope it won't happen to us, but this last year especially was scary, at times it seemed as if the whole state would burn.
My broody coop is about 18 inches up. I kept everyone in for 4 weeks before allowing freedom. The first night one of the Silkie babies needed help getting back in. After that they all had it figured out.