If Green is Venus on the basis that steamy dinosaury jungle planets are always Venus, then the Long Sun Whorl is the moon, which is riddled with underground kingdoms in the same sorts of stories.
As far as the inhumu secret: the other inhumi mystery is the use (or lack thereof) of technology. I haven't grasped what the inhumi can or can't do with technology, or how far imitation can take them (Krait and the rifle versus Krait and the boat's rigging). But, how might this fit into the secret vulnerability? It can't actually be the secret, as Krait was pretty blithe about mentioning his capabilities with both the rifle and the rigging, but it surely ties into the nature of inhumi intelligence.
Also, I can't remember what made me think this, but when reading Blue, I kept thinking that Horn wouldn't be the narrator of the next two books. I think I decided that Sinew would write Green. I'll eventually get around to rereading and see if this was just ramapnt second-geussing on my part, or if I was actually picking it up from the text. Did anyone else have this impression?
cephalothorax
*This is WHORL, for discussion of Gene Wolfe's Book of the Long Sun.