Question: If Kathryn doesn't really care about Sebastian (she thinks of him as "just a toy"), why does she even care if he falls in love with Annette? I thought she was more concerned with Court.
Answer: I agree with a suggestion made in the other answer: that Kathryn does not want Sebastian to love someone else. I've known people like this. They reject someone but enjoy being desired by that person. Then they become jealous when that person is interested in someone else because they don't want to lose the admiration. Kathryn does not want Sebastian to change his ways and start a new phase of life with Annette.
Question: Maybe I'm just missing something here, but when Kathryn and Annette talk in the bathroom, why does Kathryn act as though she doesn't know Annette? She obviously knows who she is as she has seen her several times. We know why Annette pretends not to know Kathryn but this bit doesn't make sense.
Answer: Well, this is my own opinion, but I think that Annette and Katharine were just pretending not to know each other. Remember, Katharine doesn't know how much Annette knows and Annette knows what kind of deception Katharine is constantly pulling off. I also think that Annette uses the line "turning to God" to foreshadow (and maybe scare) Katharine. Naturally, it didn't work until the journal got out.
Question: Were Reese Witherspoon and Ryan Phillippee married before or after this film was made?
Answer: After. They met at Reese's 21st birthday party and subsequently worked together in Cruel Intentions. They wed on the 5th of June, 1999, which was after Cruel Intentions was completed.
Question: When Annette tells Sebastian that she doesn't trust herself with him, what exactly does that mean? Does she feel too tempted to sleep with him, or is it something more?
Answer: Just that. She wants to maintain her virginity, but is so attracted to him and wants to have sex with him so badly that she feels like she'll let that desire take over if they're together.
Question: Were Katherine and Sebastian's parents even still together? And where were they? I'm guessing the movie was set in the summer and they were wealthy, so had taken off for a long holiday.
Answer: We the audience are never told one way or the other where the parents are. But according to Katherine, Sebastian's father is diddling the maid (or something). Meanwhile the mother, either one, is either, a) dead, b) in rehab or c) as you say on a permanent vacation.
Question: How was Annette able to get Sebastian's car after he died? Kathryn probably wouldn't give it to her.
Answer: Sebastian could have had a will and left it to her. Plus she was likely either in jail or rehab after being caught with cocaine, leaving her with little power to do anything about it.
Answer: Either she does care about him and merely calls him a "toy" to conceal her true feelings, or she is jealous at the thought of him loving/wanting anyone more than her. Either way, she manipulates the situation to ensure he and Annette do not end up together. This is the same thing that happens in the novel upon which the film is based, "Dangerous Liaisons."