Here's my theory as of now. The abdomen half of Bowser's shell is a relatively hard plaster that is based on the skin underneath. The spiky half is built for defense and does not reflect the skin underneath.
My theory is that everything in the Mario world contradicts everything else and nothing is ever coherent, so if you want those shells to be removable, you can just do that. If you want them to be attached to their spines and thus unremovable, you can just do that.
If you want to do anything and it makes no sense, you can just do that too.