*Snip*
I'm not sooooo very sure about Coryphelitis being a romance option... but... whatever floats your boat, OP.
I fully agree with your plot-relevant comment about the option to side with him or to take his place, though. Been saying that since I finished the game months back, the one-way-exit is simply atrocious and not worthy of having the BW seal on it.
We've never really had the option to side with the Big Bad. Unless you count the Indoctrination theory or Meredith, but that's hardly the same thing.
You couldn't start worshipping Urthemiel, or take Saren's offer to join him, or say "Sure Harbinger, I'd love to go with you," either.