The child safety lock would only work on the rear doors, IMO. On our Chevy you can also lock the windows, probably that way on most. But IF the passenger door was disabled, IMO, he would have had to "modify" it.
I wonder if his sister would have noticed of this? He probably kept her at a distance so she wouldn't notice anything, including stains, holes, or odors.
Has it been completely ruled out that Brandon may have been involved in the burglary of the pressure washers and truck parts? Knowing what we know now I can see how that would fit.