Anyway, anyone with a passing acquaintance with the night sky would know that something was off,

Well, would people assume Vermeer was absolutely painting the truth, or just his version of the truth? Van Gogh's "Starry Night" is the example I'm thinking of here. I know VG and Vermeer didn't paint in the same style, but Vermeer did use exaggerated perspectives if I recall.

And then there's the question of why someone setting out to fake a Vermeer would bother to paint a recognizable set of constellations in the sky, but also put that honking obvious supernova in there.

That makes no sense at all.