NJ writes:
At the same time, if she really was rotting away underneath the layer of wax, I assume a layer of glycerin would seep through after the cells die.
I don't think the mask is applied directly to the skin. I think it's a fully shaped sculpture that just sits over it like the egyptians used to use. Underneath she will likely have a face not unlike that of other mummified bodies - blackened skin, shrunken eye sockets and nose.
If cosmetic was applied directly to the skin only she would look like a mummy with blusher on!