I am not ready to convict on long-range premeditation.... really depends for me on two things: Gannon's state of well-being or lack thereof at 10:15 that morning and whether TS took the truck because it fit with a plan.
I CAN see a decent stepmother taking a child who is home sick with her, to run errands, above leaving them home alone. Quick errand, would you leave them asleep or resting in the car while you ran into a store? If a child were truly sick, but sleepy from Benadryl or the likes, asphyxiation from vomit is a real thing....
Is it POSSIBLE she was at an errand site for way too long and, in a moment of sheer self-preservation and utter callousness, decided to disappear him rather than own up to supreme negligence? Fortunately car upholstery will tell that story as will a phone, when it's on and when it's off. If the phone was off at 10:15, we have our answer regarding premeditation.
No matter what transpired, there's a long list of crimes here....
JMO JMO JMO