One other small thing that doesn't make sense: we were told that the whole reason Samantha got a job in this coffee shack was to make money to pay off a new truck she had just bought. Well then why did she need a ride at all? If I just bought a new truck I'd be pretty keen on driving it any chance I got. Maybe she let her boyfriend use it sometimes though, including that night? Anyway it's not a big deal but just something that hasn't made sense to me from the start.
Regarding the boyfriend and his reactions that night, there are a number of possibilities.
First, we don't really know anything about their relationship or the victim, so maybe this was not an uncommon occurrence. It could be that they had a rocky relationship and when the boyfriend was late to pick her up, he figured she got pissed and got a ride from someone else. In a scenario like this her boyfriend might have been ticked and perhaps slightly concerned, but not so afraid as to alert her dad and/or the police.
A second scenario is that he doesn't live with her and her dad and simply assumed that she had gone home. This would explain why neither he nor her dad did anything that night, because perhaps assumed that she was with the other one.
Another scenario, which is admittedly getting a little out there, is that her boyfriend and her dad immediately had an idea of what might have happened to her and why, but did not want to involve the police (at least initially). This would explain why it was the people from Common Grounds who actually reported her missing.
At any rate, it is worth mentioning that the police have said her boyfriend is not a suspect. Whatever his recounting of the events that night was, the cops were apparently satisfied with it.