He was taking a risk when he decided to murder his family. Criminals take risks, it's an occupational hazard.
Maybe. He certainly took risks when he handed the keys to WHF to AE and then 'fessed up' to JM shortly before disposing of her.
There was blood smears on the rifle which would indicate that the 'perp' had blood on their person. It only takes a small splash of blood to link a perp to a crime and whether there would be evidence of not - the fire pit and the destruction of June and Nevil clothes, was an ideal time to destroy any clothing he may have wore.
The blood stains on the rifle were typical of patterns from medium impact spatter eg the likely beating NB sustained from a "blunt instrument".
Agreed TODAY the tiniest biological stain invisible to the naked idea can link a perp to soc but that's TODAY using DNA testing. At the time of JB's trial blood serology testing required a large good quality blood sample. As EP destroyed SC's nightdress we will never know what secrets DNA testing might have revealed.
The blood stained rifle was found resting across SC's nightdress/person. Unless JB waited for the blood to dry or speeded up the drying process with a hairdryer or the like one might have expected to find some blood transfer on her nightdress.