For the first two mouth/jaw shots, Nevill was lying prone in bed with head raised and facing the shooter who was obviously in a higher position at the foot end. No blood on the bed clothes as it was too early to start flowing enough, when he realised what was happening and had to get up sharpish.
For the other two shots, if he was sat on the bed about to get out, the first bullet hitting his left elbow which caused the arm to jump upwards leaving the shoulder in position for an almost vertical second shot (Samson's patented arm-jerk reflex theory). Or he might have been holding his left hand up to staunch the mouth wounds which would bring his arm up to a similar position.
Wouldn't it just be easier to go with puglove's theory that NB was outside the bedroom before JB entered? Most other guilters: Clive (I think), MikeG, Scipio and yourself have suggested NB was in various positions to account for the trajectories and seem happy to dismiss the lack of blood and location of casings doesn't fit with NB shot in the bedroom. NB shot on the landing and main stairs, or even all 4 on the main stairs, does also have a guilty scenario:
Landing and Main StairsJB enters twins room and shoots them at least once.
NB hears a noise and leaves the master bedroom to investigate he gets as far as the bottom of the main stairs
Meanwhile JB enters the master bedroom via box room and upon finding NB missing shoots June x 5
NB runs back towards the master bedrom and is situated on the landing where he sustains the facial shots then turns and sustains the shoulder and elbow/chest graze on the main stairs
Main StairsJB enters twins room and shoots them at least once.
NB hears a noise and leaves the master bedroom to investigate he gets as far as the bottom of the main stairs
Meanwhile JB enters the master bedroom via box room and upon finding NB missing shoots June x 5
NB runs back towards the master bedroom and is situated at the top of the main stairs where he sustains the facial shots then turns and sustains the shoulder and elbow/chest graze on the main stairs
It may not sound as plausible as SC but it is possible. Interestingly Dr Vanezis thought the twins may have been shot first (post #2). He was the only expert to enter WHF so had some knowledge of the layout. My guess he has had time to think about the trajectories and lack of NB's blood in the bedroom.