I know this subject has DRAGGED ON but I think we really havent got anywhere near closure. However I believe I've come up with a pretty solid theory on the Shiver/Ravanna relationship
I think that Ravanna might simply be Shiver's dream self who comes out only at night while Shiver is sleeping/dreaming (like the beta-shades). After all both Ravana's 2nd name and her nocturnal nature seem to point to this. In addition, Shiver was adept at magically manifesting her dreams on the physical plane (Whisper/Nightmare Dream) so isnt it possible that at night, while unconcious (and thus not in complete control of her powers), her dream self (Ravanna) could physically manifest itself? Perhaps she was insulted by the Deceiver's comment as her lack of control seemed like a weakness to her.