That scenario makes a lot of sense to me, that he ran to get something else out of the car, but stepmom was already gone (time difference 8:45 when she left to 9am when he was last seen), and somebody grabbed him.
I don't think Kyron just wandered off. Descriptions say that was an unlikely behavior for him, and I think his focus that morning was on the excitement of the science fair, so he would have been geared to staying inside the school.
But I can see in that same excitement, that perhaps a part of his project, or a forgotten lunch bag, or maybe a prop or costume part for the talent show was forgotten in the car, and he tried to dash out to retrieve it.
I'm convinced this is an abduction, and that a predator took advantage of the unusual number of people, distraction, and lack of security at the school.
I think the predator is someone from the area, possibly a recent school employee or volunteer or contract worker. I base that on a familiarity with the lack of school security.
I hate to doubt a child. I really do. But I'm not sure the other child saw Kyron in the late morning. I'm not dismissing it completely, because again, I just hate to doubt a child. I don't think he was lying by any means, but may have been mistaken about who he saw, or what time he saw him.