Your commentary on their appearances mirrors my own, although I have two things to add.

Asriel: Needs darker hair. I'm not sure why, but but he's not right in my head unless he's got black hair.
Mrs. Coulter: I believe Philip Pullman actually cast Nicole Kidman in his head when he wrote the books. However, I want her in red. Bright, flaming red. She was always in red to me ... did he mention that, or was that just me?.