Theories. Based on the assumption that TS is the perpetrator, when I think about what may have happened, I try to order my thoughts two ways:
TS alone and TS with help. And then which series of events could actually have happened based on time, physics, or "believability".
TS alone would be unable to do many of the things that have been suggested.
Fact: We know she was gone on Monday between 10 and 2.
Assumed: She left with GS and came home without him - from what we can see second-hand on the video.
Assumed: There is no relevant video before or after this time frame.
We don't know what she did for those four hours - did she do or plan something days before (premeditated) or days later (covering tracks)?
We don't know that anything over the weekend caused her to do something to GS. Is it believable that she went hiking with the kids the day before she killed him? Would she kill her stepson over a ruined carpet? Over something he said ("I hate you", "I'm gonna tell")? Or is she actually a sociopath who was triggered by a child's behavior?
--Could she have killed him during that four hour time period?
--Is it possible to dispose of a child's body within four hours in such a way that he has not been found after three weeks?
--Is she capable of killing her stepson, e.g., using a rifle/bow&arrow/running him over/poisoning/pushing off a cliff?
--If she tossed anything out of her truck window [on 105], consider the rate of speed and the direction in which the item(s) may have traveled and where they could have landed. She could not have tossed a body out of a moving vehicle: if she was driving, if there was someone behind her, if she was alone.
--Could she have carried a limp or dead 90 pound body up a trail? Not likely during midday on a Monday, not to a place where he was out of view, or where someone driving by would not see her.
--Could she have used a duffel? Yes. But if she dumped that, and someone saw her, they'd likely investigate the duffel. Curiosity and also, littering.
--Is it believable that if this was spontaneous that she had multiple burner phones, a plan, and means of disposal?
TS with help would be able to do all of the things that have been suggested.
TS alone meets with second party during the fours hours, and second party continues on with GS, and either keeps him or kills him. That opens the speculation to location and method, however gruesome. She's then available to push the runaway idea and say she did not hurt him and he is not dead. Did someone ask her about an accomplice?
Second party could be a family member, a paid stranger, a friend, or a lover.
Time frame becomes irrelevant. Second party is under the radar and has unlimited time.
Believability goes up: If she didn't kill him, she is not lying.
SAR team activity: she could still have thrown things out of the truck window as a decoy trail.
Just MOO