I don't know why we can't entertain the idea that CM used the service road behind the back fence to load the bodies and have additional access to the house from the back. He had scaled that fence before and had run up the mountain, so he knew the area.
I suggest that he murders the family in a surprise visit, leaves at the 7:47 mark and parks the truck behind the fence. He then knocks out a couple of slats, returns to the house through the back, accesses QB on Joey's computer. loads the bodies, replaces the slats and continues his mission.
I would also pose the question: If CM's truck was in the driveway and the Trooper wasn't in the driveway, where was it? Neighbors say the family always parked in the driveway to the garage.