She didn't take much from BD's? Hailey took NOTHING.
There have been NO reports that Hailey took one thing from her Dad's house.
If Hailey had gone back to one of these hypothetical sleepovers you write of to obtain personal belongings, don't you think that at least one adult would come forward with that information, that Hailey retrieved some belongings from their home?
One may travel light while on the run but one does not travel without basic neccessities and money. Boyfriend? Not one report has stated Hailey had a boyfriend and no boys are reported missing. Besides, what boy this age could support the both of them? Gas money? Hotels? Food? Clothing? Medicine?
If Hailey were with another person, be it an adult, family member or family friend, no one has reported anyone else from this area as missing, so by and large most people are in contact with one another. If this hypothetical unknown friend is keeping, now 15 year old Hailey hidden from that person's friends and family, then that is kidnapping.
Someone would have come forward by now if there was any possibility of some young girl, that should be in school, in the company of an older person, all the time. Obviously Hailey wouldn't be enrolled in school anywhere, so wouldn't the neighbors/hotel clerk be suspicious that a school age girl is not in school?
Well, the intial descriptions of what she left with included the earings, and those were subsequently found at the fathers house. That means that either LE never asked the folk there what she might have taken (extremely unlikely IMO), or the folks there are just generally confused about a lot of things. They could hardly describe anything as missing when they apparently don't know what is there in the first place.
Since it was part of the initial description you would have thought that LE would have been asking the same questions at the fathers house, and if any of those described items were there. But there has never been any indication of that, so we just don't know. I would give CD the benefit of doubt, but the earring story is just too much of a strain on credibility IMO.
Under those circumstances we don't have any reliable indication of what was or wasn't there, so who knows.
Boyfriend: IIRC she was described as having a boyfriend briefly before she dissappeared. She was not known to have one at the time she went, but, if the boyfriend was a lot older than her (in other words, had the resources to run away and support her) you would expect her to be keeping that a secret. No boys have been reported missing, but what about young men who are no longer in town?
If she ran off with and is being sheltered by some family friend or family member, she would likely be out of county or out of state, living life as normal as a relative or under an assumed named. It isn't that hard, people do that all the time.
So, there are lots of alternative scenarios.
And that is not to mention all of the other scenarios, such as stranger abduction, or accidental death among her peers doing something they weren't supposed to (and subsequently covered up).
There are lots of options, not just SA and/or BD did it.