IMO, especially if TH is involved, I don't believe whatever happened to Kyron was accidental. If this was an accident there would, IMHO, be forensic evidence. The truck would be damaged, or it would've yielded blood or other fluid evidence or would conversely be bleached out, which would be extremely telling. LE has openly stated they have no evidence Kyron is deceased, which means that they have no evidence of a murder. Considering the statistics, that Kyron has been missing nearly two months, Terri's current whereabouts are accounted for, as are her newly spotlighted friends, the likelihood that he's stashed somewhere is unlikely, and that was always unlikely, IMHO. It's much more probable that he is deceased.
That there is no evidence of a murder leads me to believe one of the following three scenarios is true -- Kyron was incapacitated at the school and carried out in a container, thus containing any forensic evidence; he went willingly with Terri and was incapacitated/murdered outside the vehicle and was disposed of within walking distance of the murder scene; or that there is no forensic evidence in Terri's vehicle because she didn't abduct and/or murder him.
Regardless of how he exited the school, if Terri is involved, it had to be premeditated, IMHO. Unlike most here, I don't believe this has anything to do with money. She's not his mother, and his biological mother is living. Life insurance on children is typically minimal. Anything involving offing Kyron and then Kaine in order to be the sole beneficiary of his life insurance money is just too convoluted for me. For forty years this woman lived a rather ordinary, unremarkable life. Yes, she got a DUI a few years back, but it was an isolated event. She has a college degree, and therefore the means to provide for herself if need be. She doesn't strike me as the type trapped by no financial choices. She's divorced two other men, so she doesn't seem to have trouble letting go. There are just too many ifs and the payoff for Kaine's life insurance wouldn't be for a long time. That's just way too messy, IMHO. Therefore, I don't believe money is a motivating factor.
If you believe the MFH plot, then it's unlikely Terri viewed Kyron as a threat to Kaine's affection for her. It doesn't seem like she would care much about Kaine's affection for her if she was willing to kill him and lose his affection altogether. Jealousy is a possible motive, but an iffy one for me, regardless of the veracity of the MFH plot.
Pictures can be deceiving, but Kyron appears well-cared for. I doubt Terri was physically abusive, or at least not overwhelmingly so, because Kyron regularly visited Desiree and Tony. There are no reports of abuse, no allegations of abuse, even from DY and we know there's no love lost there, and there's no investigation into prior abuse. There are pictures of Kyron eating, playing, working on projects, visiting with friends, on vacation, all very normal activities, and Kyron appears at ease engaging in those activities. It's not a popular opinion, but I believe Terri loved Kyron.
I've seen comparisons of this case to Scotty Baker, and I was unfamiliar with it so I read up about it. In Scotty Baker's case, the father and step-mother had not been together for very long and the step-mother had certainly not nurtured Scotty from infancy. For better or worse, Terri mothered Kyron for most of his life. The parallels between this case and the Baker case are extremely weak, IMHO.
Terri killing Kyron because a new lover didn't want kids a la Susan Smith doesn't make sense to me, either. If Terri left Kaine, Kyron would be a non-issue for her. Baby K would be the threat to a new relationship. So that rules out "for the love of another man" as motive, IMHO.
If Terri is guilty, then I think something happened in between the last time Kyron visited Desiree and Tony and the day he went missing, something Terri couldn't hide or didn't trust Kyron to hide. I don't think he witnessed a drug deal or her in the throes of sex with someone other than Kaine, either. It would've had to have been personal to Kyron, IMHO, and something bad enough that made Terri believe killing him was a better option than anyone finding out what she'd done.
Terri sending e-mails or discussing with others that Kyron got on her nerves or pissed her off isn't a big deal at all. Children can be brats. They test you and try your patience and make you want to pull your hair out. Her venting to a friend that Kyron was driving her batty is a normal part of motherhood, and although I'm not a stepmother, I can imagine that trying to parent someone else's child is difficult at best, especially if that child had different rules at one house, or if the bio mother disapproved of the stepmother/father's form of discipline. Even in the best of circumstances, co-parenting is very, very complicated and delicate.
Kaine's disclosure that Terri suffered from PPD and ignorance about whether or not she was still on medication, if she was truly all right, etc., makes me wonder if the normal frustrations of parenting combined with the complications of PPD led to an explosive event or series of events in which Terri abused Kyron. If she was physically abusing him, I don't believe she had been abusive for long, although she could've been mentally and verbally abusive for a while. Maybe, maybe not. Either way, if Terri is guilty, I believe PPD exacerbated a frustrating situation, she abused Kyron, and the situation devolved from there until she'd convinced herself the only way out was to kill him.