Regarding the part I've bolded. If someone vaguely knew Diamond and King, they'd know that it would probably be as easy to lure her with candy as it would be for a 2-year old, and she'd probably have some level of trust in someone who she'd seen around and maybe someone in her family says 'hello' to when they see them. It could just be some really small thing like someone who packs the family's bags in the supermarket.
It is still doubtful. I think in this scenario Diamond would more likely have been taken alone, but she's probably like a big sister to King and if it was a case of luring, then she might say "King's coming too, I can't leave him here". So King would get in a car with Diamond without making fuss because he trusts her.
In this scenario, we have to remember that Diamond has a different mental age from her physical age, and there's often a naivety that comes with that, and it puts her a lot closer to King's physical age.
Reasons for this scenario? Maybe two late teens/early 20s, high on drugs maybe, looking for some cruel thrill to have some vile 'fun' with someone with a low mental age but the body of a grown woman. King would be like an afterthought, put him in another room with some candy to keep him quiet, but when the 'fun' has been had and they come down from the drugs, there's a situation to 'clean up', by which I mean Diamond and King, who now can't go back home.
Maybe this is a silly post and pointless conjecture. But if someone is responsible for their disappearance, then I want to see closure for the family and justice for whoever hurt two innocent beautiful people. And it's such a sick feeling that there's no answers after such a long time.