Mary Queen of Scots was imprisoned by the English for about 19 years before her execution and was pretty unpopular in Scotland during that period. So it is entirely possible that her morphing into a skull was intentional.

I guess this could be interestingly image-dependent. In particular she’s quite pale, so I wonder how many surfaces could be shared between the two images.

It seems to be much simpler than that; the prisms were solid and removable. So you just put them in a rack so all of one side is flat, and paint directly on them. When that painting is done, you rotate all the prisms to the next side and do the second painting.

https://en.wikipedia.org/wiki/Tabula_scalata