You're right about nihilitas, it seems to have shifted sense since classical times. I should have been double-checking my work with a medieval dictionary. I do like inanitas.

I agree that supernus is absolute rather than relative, but I read the English version as having the absolute meaning: "Only nothingness above [i.e., in the heavens, where you'd expect gods to be, but they aren't, so there's nothingness instead]" so it seems like it fits.