LOOK at those images. It's not a CG model of Maul's face under there. It's the flat image of his face from the TPM poster. It's probably just wrapped around a simple tube or ball shape with Grievous's skull over it.
Stop overthinking things and try common sense. I know it's an old fashioned concept that's not very popular anymore but it can still work to your advantage if you know how to do it properly.