According to legend Nagash's head was meant to be a joke sculpt but someone thought it was the real one and sent it off to the mold people.

Mummies would work good as wraiths but as you already have 10 wraiths it is probably best to use them as something else. I think it would look cool to mix them into a GG unit for some variety. Lorewise that would be pretty reasonable as the GG are a mixture of dead guys from assorted ancient cultures of the old world back when Khemri was still kicking around as a non-undead nation. Makes sense that some Khemrian warriors would end up dead and hastily embalmed around the old world.