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.
They are together. Kathryn says that "the parental units called." They're in Bali, and the maid is probably a hotel maid there.
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.
Factual error: During one scene where Ronald is giving lessons, he tells Cecile to play a G major scale. After he says this, he adds, "Remember the third note, it's sharp." In a G major scale the third isn't sharp. The seventh is. (00:18:35)
Suggested correction: When playing the cello, there can either be one finger space between the two notes or two. The first G would be just the G string with no fingers. The second note would be A, which means just putting a finger in the correct space on the string. The third note is B. The third note would use the third finger. The alternative, which would be wrong, would be to use the second finger. What Ronald is saying is, rather than use the second finger, you should use the third finger.
This correction seems to validate the mistake. There's nothing in the quote to indicate he's talking about finger positioning, you're not playing a sharp on the 3rd note.
Answer: Kathryn tells Sebastian that "the parental units called," as if they are together at the place that they called from. He asks if her "gold-digging" mother is enjoying Bali. She jokes about his "alcoholic" father "diddling the maid." I think the parents called from a hotel in Bali (so she is referring to a hotel maid). They are together, even if the marriage is just a sham at this point.