Interesting, except for one thing. It doesn't describe how Ravanna, a contemporary of Myrdred, Damas & Connacht, came to be resurrected as Shiver in Myth 2 - in fact you've left this out altogether.
A while ago Forrest described both of them as ancient evils (correct me if I'm wrong Fobo). To me this doesn't make sense either, as how could Ravanna be on the side of the light in Myth III (assuming she is)?
My view of things is this. Culwyeh is an ancient evil, probably a spirit as you've described. To make her the first leveller stuffs up the whole A/B Spirit theory, so I won't go there.
At the time of MWA, Culwyeh may or may not be around. She is not, however, joined with Ravanna at this time. Ravanna is an avatara on Connacht's side, but already having disagreements with Myrdred. "That mouth of yours has got you into trouble before" definitely shows that the two have past history, and not necessarily as enemies.
Anyway, Connacht disappears, along with his lieutenants, and many years later returns as Balor, the others as Fallen Lords. Ravanna has become Shiver, and in this transformation has conjoined with Culwyeh. Thus the age-old argument about Ravanna "pretending" to be Shiver is solved. She is whammied onto the ether or whatever by Rabican, restored by Soulblighter, then finally killed by the Deceiver.
cheers,
-Iron
BTW Forrest - is that really you in that picture on the FutureMyth forum? And how's the Faceless Man coming along?