I agree with you, too, I've always had the gut feeling that her scent being at that motel was very significant. Her age at the time would be about right for what these human traffickers prefer.
The problem with that is that typically in those sorts of situations the girl willing gets involved. Usually they are runaways and get dependant on their new "friends" through shelter or drugs.
If HD was being used as collateral by someone in the family then it is certain that she went along with it. If she wasn't cooperative then as soon as she was released she was going to complain to one or both of the parents, and that would not go well for the person who used her that way. That would rule out SA in that scenario, since there appears that there was no way that HD would cover for SA like that. The only people she would do that for would be BD, CD or DD, not their significant others, not their friends and not for relatives.
Also, if someone did that, they wouldn't be going to the police immediately as the people holding the collateral would likely get a bit upset if that happened. So if that is the case you can pretty much rule out BD as having anything to do with it. If it is a drug collateral thing, it would more likely involve CD in some way, which actually would fit with all the bizarre behaviour he showed the day she dissappeared and subsequent to that.
You have to wonder if this may have happened before though, remember that at one point she spent time living with friends of her parents, which I allways thought to be very odd. If her parents were having issues, wouldn't she have been living with one or the other, or one of her grandparents, or one of her relatives? Most of them appear to live in the general area, so it seems strange that she would live with family friends.
Lastly, there is another possibility, that HD was up to something that no one knew about. She was at the age when kids start experimenting with adult things, such as alcohol, drugs and sex. If she was doing anything like that she would not be telling many people and certainly not her family. The thing that makes me think this is possible is the text message on the day she dissappeared to her friend, which read "where are you?" or something along those lines IIRC. It is the sort of question you would ask if you were expecting someone and they didn't show up. It may be possible that HD was up to something secret with some adults, involving her friend perhaps, who got cold feet and didn't come. The friend would not subsequently want to get involved or get in trouble so she would have kept quite when asked about it later. Then, when SA arrived home unexpectedly, HD quickly took off to meet whoever she was planning to meet. And if they were older with sinister plans to use her, she may have found herself in a bad situation very quickly. That might account for the scent hit at the motel.