Unfortunately, the scenerio with the step mother is the only one that makes sense right now. Until we know more, which LE doesn't seem to even know right now. Two things stick out to me. One, she is a fan of CSI, which might give her more knowledge to get away with things, knowing about evidence and what LE would first look for. Leaving him at school, having a picture and witnesses that saw them there yet no one saw her (or him leave). Also, and this may be a stretch, but what I was thinking when I read her Facebook today was
http://en.wikipedia.org/wiki/Münchausen_syndrome_by_proxy. Like I said, it's a stretch but she seems so calm and collected and kind of relishing in the attention?