She then went back home and uploaded the photos of Kyron and his friend Ks in front of each of their science projects in order to make it seem like it was just a normal day in her life. I think the fact that she uploaded the pic of Ks too means that she was trying to make everything look normal, if she was responsible for Kyron's disappearance.
What kind of human being, who had cared for Kyron for so many years, could possibly look normal (no matter how hard she tried) after she had killed or otherwise made him disappear?
Unless something is terribly wrong with Terri, if it had been a true accident and she had hidden the body so she wouldn't have to face the consequences, she could not have appeared to be normal. If she had killed or otherwise disposed of Kyron, she could not have appeared to be normal. This is a boy she knew and took care of on a full time basis for years. I don't think anyone with an emotional investment in that relationship could carry it off.
There wasn't much time between Kyron's disappearance and when the school bus arrived. Perhaps she wasn't in significant contact with other people between those two times, so she could carry it off. (Even when Kaine was home, he could have been working in his home office, while she was elsewhere in the house.) Afterward, her responses could be chalked up to shock, fear, misplaced guilt.
Maybe what happened over the past month was that Kaine looked back and saw lots of abnormal responses and behaviors over time,
to add to what he observes now that the initial shock has worn off. He may have talked this over with Desiree and her husband. They may have thought of some times when Kyron said things or reacted in a way that was unusual, as a result of SM's treatment of him.
Taken alone, each instance could be explained away. Taken together as a pattern, a picture of evil emerges. Terri is outed as a psychopath or sociopath.
This is mere speculation, and only my opinion.
Also, this is one speculation about one person. Without more info from LE, everyone with access to Kyron (including strangers) is equally suspect, AFAIK. I do think that the three other parents have been "cleared" because they have strong alibis. If the other three were at work and around other people, for instance, that only leaves one parent-suspect who was running around unleashed.
RC