What does that mean? Jeremy planned to enter the farm in the dead of night and kill his family in order to inherit early. He pretended to receive a phone call from his father in order to distance himself from the incident and implicate his sister after he staged the crime scene to make it look as though she had 'gone crazy' with a gun he 'just happened' to have left out. 'What' he did while in the house and in what order, only he knows. How can anyone tell you 'Jeremy's theory'?
If that was his plan then he must be completely mad. Too many 'What if's...':
Somebody saw him on the road.
The dog barked and alerted Neville.
Someone heard him climbing through the window.
Someone was awake at WHF and called the police.
Neville had unloaded the gun and put it away.
He was overpowered by Neville.
Sheila locked herself [and her children] in the bathroom or another room.
Sheila refused to cooperate and allow herself to be shot.
He left a tiny amount of blood on the window, the bike, his clothes.
JM went to the police.
Et Al...