It looks like we'll need to start over again with Charlie.
He's popped his belly again. He goes back in tomorrow.
This time, I'll see if we can avoid stitches, and just confine him to his cage. That collar depresses him so that it might interfere with his healing. Surely birds heal in the wild.
UPDATE: After taking a good look at him, not spotting any infection, she suggested doing just that until his keel is feathered over. He'll still need oral antibiotics, which is not a problem.