Those are good questions....Here is my theory.
In my opinion, he never cut his foot on Saturday night. He wouldn't have gone on a big hike in that rocky park the next morning, if so.
She made up that story, IMO, because she knows LE probably found blood evidence in the garage and maybe trailing out towards the side gate and garbage cans.
I think that blood was spilled on Sunday evening. ...so much emotional chaos and upset that was going on that night . And Gannon was very upset.
She kept him home from school Monday. And he did seem to be walking very slowly when he was seen going to the truck Monday morning. Was he injured, sick, sad, ...I don't know. But it appears to be the last time anyone, other than StepMom, has seen him.
My guess is that she badly injured him on Sunday night. Not a fatal injury. But if he had wounds , bruises or internal injuries, she would have been arrested the next day, if anyone saw those injuries.
If so, she would have lost her teaching credentials, lost custody of her daughter and stepdaughter, and lost her husband. And likely been arrested as well....
She could not allow any of that to happen.... She felt she had NO CHOICE but to make him disappear. JMO