You make good points and ask some very good questions.
Yes, Tracie had to work that evening and did not get off until 9:00 pm. She went home afterwards and changed clothes. She and J.B. left from her house and set out to find a certain field party. They were unable to locate it after driving around a good deal of the evening. They ended up at another field party where they put stopped for a while before leaving there and going to the Big/Little Store to make their phone calls. Tracie did ask her mom could she say out a little later past her curfew as she and J.B. were trying to meet up with a couple of boys. Her mother gave her permission and went to bed being tired from working a double shift. My guess is J.B. had called the boys from either the aforementioned party or from the Big/Little Store telling them they couldn't locate them earlier but could meet them in a few minutes in Midland. They never made it to Midland, probably because they were intercepted by their killer(s) along the way.
You are correct about J.B. being somewhat familiar with Ozark having gone there a few times with her dance team. But, it is easy to get turned around, especially when darkness limits your visibility.
You are correct about killers and rapists being charming at times. Ted Bundy was extremely charming when he needed to be. Like you, I have wondered how McCraney was able to gain control of J.B. & Tracie. He may have seen them at the Big/Little store and slipped into the back seat of their car when they were out on the phone and speaking with Ms. Merritt. Or, he may have seen them at the store from afar and stopped their car by simply stepping out in the road after they drove off and out of sight from Ms. Merritt. Also, I have wondered if maybe he was at the party they attended and struck up a conversation with them. Then, when they saw him walking stopped and picked him up?
As for what set him off...no one really knows. There a lot of possibilities. Maybe after he finished doing what he did to J.B., she cursed him or threatened to go to LE and he decided to shut him up? Or maybe the girls running from him (remember the briers in Tracie's pants and mud on her shoes) angered him to the point of murder? Since his arrest, I have always said that I wished the entire story of what happened that night would come to light-specifically, did he have an accomplice, how he captured them, why he chose them as his victims and most importantly why he killed them?
Whether he had done this sort of thing before or since is another good question. I am certain LE has revisited any unsolved murders and rapes around that time and in the past 20 years.