We do see a brunette female extra in Majel's adapted white uniform. I guess they'd have beefed up a scene, although I think the script was still being written when it was made known that she was stepping away.
Her scene with Sarek made it into the comic adaptation.
Yes and yes. (Riley was runner up.) Christian Slater's mother, Mary Jo, was casting director, so Grace's scene in Sulu's quarters were given to Christian at the last minute, although it made more sense not to have Rand barging into Sulu's quarters while he was asleep.