Red flags were waving to me when JS says she asked SS to "pick Maddie up and take her to school". You don't ask someone to "pick someone up and take them somewhere" If they lived in the same house. You'd say "can you take her to school", not "pick her up". Unless either SS didn't sleep at the house that night, but that would be odd for JS to ask him to come over and take her to school (~90 mins before it started), or Maddie herself didn't come home that night and she needed to be "picked up from somewhere".
While I could be wrong about this... I'd think LE wouldn't word it as "moving the body" if they were talking about getting a body in a car from the house. Sure, it's been physically moved because you can't teleport a body... but why say that about something so obvious (there's a body in the car so therefore it must have been moved there). It just sounds to me like he moved it from somewhere other than her house due to LE's wording of that.
I think a couple of things were happening concurrently.
First, the subhuman SS was working a years' long scam. Presenting as a steady stepfather figure to gain unfettered access to a minor, had to make sure that stayed solid. The mother, groomed and gaslit. Just like he groomed and gaslit poor MS. Why didn't she tell? Maybe she did. Maybe she was underbelieved or he was overbelieved. Maybe she didn't tell because she was told that telling would cost her her home or her mother or embarrassing photos... or her brain itself was confused, trying to make sense of abuse she'd grown up with. Both mother and daughter living with major cognitive dissonance. SS may have convinced or attempted to convince MS that it wasn't abuse but a relationship. Vile. But not unheard of.
So the other part of this is that he obviously wanted to stay on this side of getting caught, not just by the police, but by JS.
Let's pretended she is allllllll victim.
Let's say SS returns from the party. Convinces JS that MS has already gone to her room. Oh so tired. Let's say he sits with JS for a time, lets her know he's planning to head back to his mom's for the night. GIves her a lil sumpin sumpin which knocks her out. Giving him time to work with AND might explain why she woke up too groggy for chauffeurring. So she calls him, assuming he had left, to come pick MS up.
Now he begins the next charade. Has to convince JS that MS is home, alive. Or, if not that, that there's a story to tell and she needs to provide and stick to it.
IMO MS was never in her own bed that night (unless SS drugged JS). It's possible MS was murdered the night before and never went into or out of that house at all. Just fancy foul footwork for SS, messing with JS's head.
Example: JS hears the shower. Hears MS's bedroom door. Believes SS is not there, it's just the two of them, JS and MS, at home so any sounds would lend, based on the errant assumption, toward being from MS. (Let's hope she's being utterly forthcoming with LE now.)
FWIW I think he murdered MS after the party. Went home to arrange what needed arranging, including JS herself. Returned once to discard the backpack and tablet, almost an afterthought. Drove MS toward school. Her body that is. Idiot attempts at staging normalcy.
JS may be an accessory or she have genuinely and blindingly believed that whatever happened to MS, it happened after 8:30 and the events from the morning were irrelevant... and also fuzzy. By design. His.
Lots of layers to pull back.
Meanwhile SS had how-many-unsupervised-hours to do his deed, clean up/conceal/stage.
Sadly I don't think MS made it out of her birthday (birthday-party day).
And if I could, take it one step further, perhaps MS had been told something "special" was going to happen after her 13th birthday. In the woods. And it was SS who set that up, knowing precisely what evil would overtake her there. SS controlled the subplot of this whole family.
I hope the moron had his phone with him the whole time.
JMO