The absence of blood stains pertaining to Nevill is easily explained as he was standing approaching his assailant who retreated backwards out of the bedroom. We know that blood does not instantly escape from a gunshot wound so if Nevill was standing up while moving forward then the blood would trickle onto his clothes and not onto the floor.
Blood was identified on the wall by the main staircase which was consistent with Nevill staggering downstairs to summon help. Far from claiming that it simply doesn't work, it works extremely well in my view.
I also think that Jeremy Bamber retreated ahead of Nevill when he unexpectedly ran out of ammo and that it was during this period that Nevill followed him down to the kitchen where the fight and fatal shots occurred. I also believe that JB went back upstairs and shot June again as she lay injured.
Unexpectedly ran out of ammo? This in a carefully preplanned run of executions?
I see 4 casings outside the bedroom, and any variance might be a kick from an officer during the crime discovery. A crime reconstruction requires a feasible explanation for most things, and this is all explicable by Nevill hearing Sheila making a noise downstairs and Nevill heading down, whether fearing an intruder or knowing it was Sheila.