In every case, there is what happened, there is what was supposed to happen, and what was also happening. Untangling that IME is the challenge.
What was MS's expectation? Did she know SS was returning? Did she know he was going to "take her to school"? Did she know that everybody was going shopping after school? Who did MS expect would drive her home after the birthday party? Did she GO home after the birthday party?
Did someone have a hotel room?
We are missing key dialogue. If JS was working late the night before, did she ask SS to come to the house? To babysit? Did she say she was tired? When was her next scheduled employment? Did she stay at a hotel for the convenience, was she going to be working that next afternoon/night? Did she think of it? Did JS say he'd drive over and get MS situated and off to school so JS could sleep and not be troubled with the task? Perhaps the hotel was farther away from the school than home, making it a more lengthy task, and JS convinced her to stay there.....
Was SS planning to be at the house that day/night? Planned or spontaneous? Whose idea? Why did he drive his father's car? What was SS's plan/expectation for his next 24 hours?
Blaming MS for the phase she's going through burns me. It's not true. And it goes toward chronic gaslighting IMO. Blame the child. I'm guessing that ran deep with him. If the car were the issue, why not drive JS's car? (I don't think it was an issue, I think he made it up to account for why there was no video of him dropping her off AT SCHOOL.)
If JS was home that whole time, did SS have anything to do with her profound tiredness?
At what point did JS know he was going to murder MS? Was he done with her and drive there with that purpose in mind? Did she die of violent SA, held down. Asphyxia?
Then SS, "I tried to save her... but she didn't make it."
Asphyxia. Choking. Choking where he went too far that time. Blunt force trauma to accurate the inevitable? How many cases have we read where the murderer can't handle the death rattle and grabs an implement/weapon to silence it? (It's always always always all about them.)
From there it was less about story building and more about concealment, but just utterly haphazard.
His return trip to the development, I can't make it make sense. Unless.... unless his initial plan was to dump her body by the school somewhere, went extra early but was thwarted by too much activity. So he returned to the house, where he then moved her body to the trunk. Waited there a while before abandoning the school idea altogether, ultimately settling on distance. If he hasn't gotten a flat tire, how far would he have gone?
IMO though we might be in the dark, LE has answers to much of the above.
JMO