Like most of you, I think TH is guilty, and quite honestly just based on the first images I saw of her in the news I knew she was a Susan Smith. Here's how/why I think it went down:
1) TH developed hatred of KH for whatever reasons (he was controlling, didn't like her son, paid more attention to Kyron than they're daughter, etc).
2) Tipping point was when KH kicked out her son. Her simmering hatred turned to all engulfing rage - she thought, you take my son I'll take yours.
3) She plotted to get rid of both KH and Kyron.
4) MFH landscaper fell through, but she was always going to get rid of Kyron as well so thought she'd like to see KH's face when Kyron went missing anyway (as at this point hurting KH was as important to her as getting rid of him).
5) She methodically plans out different means of getting rid of Kyron, gets supplies over time and mentally goes over the best time/day to do this. The science fair scenario was only one of her plans, but there were others.
6) Over time, she works on getting a sympathetic friend to her plight - enter DDS. She tells DDS about KH, how abusive he is, etc. and DDS offers to help.
7) The science fair was a premeditated day to go through with her plan.
8) She takes the truck because it can go 4-wheeling off road.
9) She gets to the fair early - like 8:15 AM early. She makes certain she is seen at the fair and does the 'appointment' scenario of confusion with the teacher, realizing how busy they'd be that day and how easy it would be for a teacher to 'mis-hear.' So far, everything is going according to plan.
10) She scopes out the crowd and takes Kyron back and forth to her truck during the science fair to test out if people see her. She also shows him where she'll park later.
11) She tells him to meet her off the side entrance for their appointment, reminds him where she'll park the truck and tells him not to be long and not to talk to a bunch of kids but come quickly.
12) Kyron comes out and gets in the truck, as planned (around 8:45, not 9). TH makes sure no one saw them (otherwise, she would hold off on her plan for another day).
13) TH goes to SI and hands him off to a SO, murders him or gives him to someone else to hide (if a person were to hide him (not an SO), they would have to be convinced KH is abusive and they were saving Kyron). If she murdered him, sadly I think she would make it look like an SO by doing something to him that is typical of these monsters.
14) She calls DDS panic-stricken that something happened to Kyron on accident, or that she did something really stupid with Kyron (handed him off). She begs for DDS to help her.
15) They meet at a Fred Meyers parking lot, where DDS follows her to the crime scene - DDS either helps dispose of the body, helps cover her tracks, or gives her advice on how not to be suspected if she handed him off. DDS is convinced by TH that none of this would have happened if KH wasn't such an )(*$# hole and TH should not do time for this (even if she knows TH killed him).
16) TH goes to the gym to change her clothes and get rid of any evidence. It is quite possible she throws away the clothes she wore that morning.
17) She makes sure to post pics of Kyron online, to write the teacher about picking up the project, etc - much of this not just creating alibis but also 'feeling' people out, as in if she wrote the teacher and the teacher immediately called and said 'we've been trying to get a hold of you, where is Kyron?!' then TH would have to think quickly about what to do in this instance, versus making sure she looked surprised when he wasn't on the bus.
18) TH goes to the bus stop with KH, enjoys watching him realize his son is missing.
19) Poor KH, like many engineers, is book smart and people stupid and didn't see this coming a mile out - he rallies behind TH as she convinces him the media is evil and out to get her. She implores with him to stay away from the media and that they are trying to make money off his little boy. He falls for it and is distant with the media.
Little does he know she is still planning on killing him after the dust settles on this case...
That was the how's, here is my theory on the why's:
I believe TH is a sociopath. I believe she justifies taking Kyron because KH 'took' her son. I believe she convinced DDS of this as well - that what happened wasn't her fault but KH is to blame.
I also believe that she doesn't care if people know the truth, that she did something to Kyron, but that she feels strongly that even if it's clear as day she's guilty, unless the LE can PROVE she did something, they can go to _)(#$#$#. I've met people like this before and there is no guilt, no remorse, and a sense of entitlement along with the thought process of 'if you're too stupid to figure it out, then so sad too bad, I get away with it."
I hope she handed him off and that there are twists and turns to this plot that involve her hoping to receive a ransom that KH would pay to an accomplice, or being the hero somehow, or whatever, but it might be pretty straight-forward. The big thing in favor of a 'hand-off' of some sorts is that she did an awful lot of driving that we know about (i.e., I'm not taking anything she says for fact, but the LE's time-line shows a lot of driving) -- Skyline School, Fred Meyers in Hillsboro, Fred Meyers in Beaverton, the gym, possibly SI, and it would be faster to do a hand-off than a murder - unless of course this was really well planned and scouted out...
Whew - the above is IMHO