Hide glue is very brittle. All it takes is a bit of shock loading to break the bond. A few light taps on a chisel inserted in the joint between the rib blocks and the form are all it took to loosen the rib garland. Pulling out the form was then simple.
I rounded the corners of the end blocks with the same chisel, then sanded the entire interior perimeter. Next week I'll glue the back on, and from that point I will be back on the "proper" sequence.