Well all I can say Adam is if the rifle was inches away for the first shots to Nevil the shooter was a very poor shot. Bamber would not need to get this close anyway, I think you have solved the case the shooter had to be Sheila
This wasn't a carefully choreographed and rehearsed scene from a Mission Impossible film.
Bamber would have been in a very heightened state, that would be a mixture of emotions, all fueled by adrenaline and anything else the body pours out into the blood stream in moments like this.
Not even a psychopath can stop the release of adrenaline and other chemicals that change the composed way that someone may normally behave.
I think Bamber probably only planned the order that he would kill his family, but he had no plan B or plan C. Given his known behaviour in the years leading up to the murders, he wasn't much of a forward thinker.
When Nevill woke up, he would have tried to disarm Bamber, and therefore chaos ensued. All bets were off.
It is no surprise that in that heightened and frantic environment Jeremy Bamber lost full control of the situation, so he was - for example - unable to place the gun carefully and slowly between Nevills eyes and pull the trigger.
But he was able to hit his target. He was trying to murder a moving target who was bravely trying to defend himself, his family, whilst trying to disarm Jeremy Bamber.
Given all of this chaos, the injuries as they have been observed and documented perfectly fit with an experienced shooter, who was faced with a victim who was moving and fighting back.