My theory is very similar to yours (NK mastermind), except in mine, I think that DE "inadvertently" became a pedophile. What I mean by that is that, I think he reached out online for someone (not specifically targeting a kid), and found Nicole who was very receptive. He wasn't specifically targeting a child. Maybe he figured it out on his own later on. But, even though he may have not known she was 13 initially, he groomed her anyway, and used her for what he wanted (nude pictures, etc).
Then, the rest of my theory currently goes something like this: After he got what he wanted, he got bored. He wanted to move on. Or it is possible, he figured out at this point that she was a child and thought, "oh s**t, what did I just get myself into"). Nicole in the meantime, thought she was in a "relationship". She's 13, and longing for a boyfriend. She talks about running away with him. She thinks he is really into her since she shared something so intimate with him and doesn't realize he does not feel the same way. Nicole kept pursuing him on line. He didn't know how to shake her off. DE makes a comment about this to NK ("like what is this girl's deal, how do I get rid of her?"), or possibly NK was snooping on DE's phone and uncovers messages Nicole is sending DE. NK starts doing her own sleuthing on Nicole. And what she discovers reminds NK of her own 13 year old self and she is disgusted and hates what she sees. The similarities are too stunning, and it is like reliving her own 13 year old troubles all over again. She may possibly even hate that Nicole is communicating with "her" guy. She starts to come up with a plot. But the plot she shares with DE at Cookout isn't necessarily the same plot that is going on inside her head. Maybe it is the medication making her think strange thoughts. Maybe she is just evil. But, possibly she tells DE that she'll talk to Nicole. Maybe she tells him she'll confront Nicole and pretend that she's DE's girlfriend and Nicole better back off. And then she throws in "oh, by the way, can we stop at Walmart. I need some stuff." So, DE gets on board with agreeing to arrange a get together, and they go shopping. Nicole meantime, ever hopeful, thinks it's a date, and doesn't see what is coming. And, once DE realizes what is really going on in NK's plan, it's too late. He's in too deep. Can't go to the police now. HE was the last contact. It was HIS car. He picked up Nicole that night. NK and DE are probably on video footage at Walmart together. And, there's evidence HE had prior inappropriate/sexual contact with Nicole. In his mind, he's trapped between a rock and a hard place. He feels the better alternative of the two is to proceed and try his best to help clean up etc.
I just don't see a scenario (B) where he was the mastermind or manipulator. Based on my impression of him, I don't think he would either want or need NK to get involved. He strikes me as, intelligent, arrogant, and a bit of a perfectionist (I'm basing that off that HS video interview of him). I think he physically could have handled the task of murdering and moving Nicole all by himself. He's an athlete after all. I just don't think his arrogant mind, and his quest for perfection, would let anyone else be involved if it was his desire all along to lure and/or murder Nicole. I'm pretty sure he would not risk the potential for a screw up by bringing in someone else. I don't think the idea of killing Nicole was ever his idea. He had to know too many things would immediately tie him back to Nicole. I think he got manipulated into it by NK. I do think, though, that both are responsible for the murder.
ALL just conjecture on my part. And I'm sure there are some flaws in my theory. But that's where I am at now.
