But if the perp went to these lengths to ensure NB was dead why not do the same with the other victims? What made the perp need to single out NB?
well the most obvious reason would be because Nevill put up a big fight and he didn't want Nevill waking up and fighting him again or waking up and grabbing a shotgun and appearing with a shotgun as he was taking care of the other victims.
But he made sure June was dead by firing a shot between her eyes. He fired 2 extra shots into Nevill's head to make sure he was dead and fired one between June's eyes to make sure. He fired multiple rounds into the heads of each boy to make sure they were dead.
Vanezis wasn't positive the injuries to his back were burns or that they definitely were from the night of the murders. It has been shown that injuries from being struck by objects or a body hitting into objects have been mistaken as burns. The only way to ensure something is a burn if to test the flash for sure signs of being burned. No one records Vanezis doing such.
Thus we don't even know they are burns. What if during the course of struggling Nevill hit his back into something? Jeremy could have hit or burned him with something to try to give the impression Nevill was being marched at gunpoint or forced to make the phone call to Jeremy at gunpoint. Someone here- perhaps you- posted about burns that can be caused by bullets that land on flesh. I looked more into that and it does occasionally happen, for all we know the bullet that grazed him ended up inside his gown bouncing around hit back. There is not enough information for us to figure out whether the marks were incidental or done on purpose for some staging purposes. Vanezis not really caring much about them and thus not doing anything to truly document their exact nature means we have no hope of even knowing whether the skin was actually burned let alone to figure out what precisely caused them.
You talk of friction burns, stabbing the rifle really hard into a back could perhaps cause such abrasions. Post mortem it is very easy to damage skin and it will look worse than would be the case if the person were still alive. There are ways to assess whether injuries were pre or post mortem but it requires great skill and care. Clearly Vanezis either lacked the skill to attempt to assess such or had no desire to bother.
Only if someone gets Jeremy to talk will we ever know what caused the wounds and why they came about.