The 'forensic' evidence is it was human blood in the silencer. Certainly Sheila's according to the judge.
An awake an out of bed Nevill is much more likely to confront Sheila upstairs. Rather than run downstairs to phone Bamber
. Leaving Sheila upstairs with a loaded rifle for several minutes.
It's doubtful Nevill punched Bamber. Nevill had been shot 4 times & was bare footed in pyjamas. Bamber had the rifle which he hit Nevill with. So hard the stock broke.
As to the blood in the silencer, that's not what I have read, its highly contentious if it is even human, when you shoot a rat or fox often the first thing I person will do is nudge it with something ie the barrel.
All assertions of NB reactions are just opinion, but as said it might / would not matter who was holding it, the bullets are the issue not the holder.
And the stock could have been broken post mortem.