The Amy Sutton from Sweet Valley Twins and the one from Sweet Valley high are different people.
The books have a lot of girls that look just like the Wakefield Twins and pretend to be them on occasion (Margo being the most notable example). Who says that only Elizabeth and Jessica get doppelgangers? That could explain why Amy underwent such a drastic change between the two series. In Sweet Valley Twins, she's a smart and lovable tomboy and the loyal best friend to Elizabeth. In Sweet Valley High, she's a shallow, boy crazy bitch. Maybe somebody (Margo?) found a girl who looked like Amy and sent her to Sweet Valley to spy on and take down the Wakefield twins. It didn't work because the replacement wasn't very smart and was more interested in hooking up with hot Sweet Valley guys.
Ken's car accident in "That Fateful Night" wasn't actually an accident. The mastermind warned Faux-Amy to stay away from Real-Amy's middle school boyfriend because he might get suspicious. She couldn't resist the hunky football star so the mastermind arranged for him to get into a car accident in hopes of getting him out of the way. It worked when Faux-Amy lost interest in Ken as a result of his blindness. The scheme is discovered after Sweet Valley High. Faux-Amy is sent to jail while the real Amy is freed and decides to stay far, far away from Sweet Valley and the Wakefield twins (explaining why she wasn't in Sweet Valley Confidential).
- Not really. Amy was introduced because the writers wanted to create a link between the main high school series and the middle-school Twins series, which was launching at the same time Amy was being introduced. The idea of her being a bitch in the Sweet Valley High but a good girl in Sweet Valley Twins, was due to the writers wanting to play around with the trope of the long lost friend by having Amy be the type of person who was your BFF as a kid but when they moved away for a period of time, morphed into a complete and total asshole and come back into your life irreversibly changed and now your mortal enemy.
The current Sweet Valley High series "The Sweet Life" is a massive attempt at damage control after the backlash over "Sweet Valley Confidential"
- The endgame will be to restore the status quo by the end of the run. Winston will turn out to have faked his death and come back, having learned the error of his ways and become a nice guy again. His return will probably coincide with Jessica and Todd divorcing, as Jessica and Winston will finally hook up and find happiness. Todd will in turn save Elizabeth from Bruce, who will revert to his evil ways and may even turn out to have been been holding Winston prisoner and the one who actually faked his death so he could steal his fortune.