I agree that Andrea is likely the result of Korby's early work with the machine. However, is it possible Andrea was a member of the expedition as well?
You make a good argument that she was. One wonders what sort of an expedition it was originally... Supposedly Korby's party was dropped on the surface to follow some sort of an obscure lead, stumbled around in the icy wilderness for a while, and then signaled the discovery of underground caverns and subsequently vanished. According to Kirk's log, the original expedition was at the point of perishing and only survived because it found the caverns.
So we might speculate that the only three survivors, Andrea, Brown and Korby, crawled into the caverns, found the miracles, and then quickly created android bodies for themselves before the flesh ones perished from the original exposure effects. But I'd still like to think that Korby wasn't mortally wounded yet when first entering the caves or when creating Andrea, because it would make more sense if he remained coherent, able-bodied and free to experiment with the machinery.
Timo Saloniemi