While there are a number of reasons to suspect TH --- (i) she was the last to see Kyron alive, (ii) her account of driving around town for an hour or more seems a little dubious, (iii) her FB posts are a bit odd, (iv) her sexting with the friend suggests psychological instability, (v) her failure of at least one lie detector test, (v) her alleged hiring of the gardener to kill her husband, etc. --- there are a number of reasons to be skeptical about her as the culprit:
(i) What, exactly, was her motive supposed to be? She disliked Kyron? Sorry, but that's not enough for murder, especially from a woman who had clearly spent a lot of time and energy raising the child.
(ii) If she killed Kyron, where and when did she do this? There was no evidence of foul play in the car. How did this housewife manage to kill a child without anyone seeing & without leaving any evidence behind? Sorry, but I just don't find this very likely. Now, if she strangled the child, that would leave no forensics, but that leads me to a third problem.
(iii) How is it that his body has never been found? She couldn't have gone far, and she isn't a professional. I find it hard to believe that she was somehow able to hide his body in broad daylight (without going terribly far) in such a way as to this day, no body has ever been found.
Thus, while I agree there is reason to be suspicious of her, I don't see how you overcome the three problems above.