The challenge ratings can be altered by the situations as well. The addition of bad terrain or other items to make the fight more difficult for the characters can greatly effect the CR for any monster. I have encountered a Great Red Wyrm and I will tell you it is not fun.
The demons are more powerful, but because of their chaotic nature they are less effective in the Blood War than are the Devils who are very disciplined and act in concert.
Elminster and Demogorgon are unique characters. Elminster should not have CR since he is basically on the characters side and a motivator.
2nd edition AD&D didn't have CR's. They simply had experiance points for each creature and you added them together.
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The line between good and evil is razor sharp. Be careful of misteps,
lest you find yourself spitted upon your own blade.
[This message has been edited by Wulfere (edited 10-10-2001).]