Hey Brian,
Have you thought about supporting the crown with the mortar board hanging free, wetting the leather, inserting a plastic bag into the shell, and filling the bag with hot silica sand warmed to a specific temperature from your stove? Seems like that might work and slowly allow the heat and moisture to slowly push the form back into shape. Remove bag of sand, reheat and reapply as needed.
If worried about the outer shell splitting, perhaps form a cofferdam made out of plaster of Paris form to support the whole thing from the outside? Make it like Tony showed with his plaster cast helmet shells for displaying helmet covers, but split the back of the casting and support the entire thing with duct tape around the outside to firm it up.
That's what I would try; it would press the entire thing in between a formed and a flexible solid which would allow you to control the temperature and weight while it slowly pushes back to true.