The following was part of the prosecution case at trial:
vii) The appellant's account of the telephone call from his father could be proved to be false for the following reasons:
a) His father was too badly injured to have spoken to anybody;
b) The telephone in the kitchen was not obviously blood stained;
The prosecution case at trial is that JB entered his parents bedroom whilst they were in bed and opened fire, therefore how could NB have made the call when the pathologist said the injuries sustained upstairs rendered him incapable of purposeful speech and why wasn't the phone blood stained?
The physical evidence shows NB wasn't shot in the bedroom but on the landing with the perp already in the bedroom shooting out onto the landing.
I don't think that was the prosecution case exactly Holly, the point about NB being too injured to call JB at Goldhanger is moot if JB is in the bedroom shooting him!
I do like your idea of NB first being shot on the stairs as he realises his folly in turning his back on SC to phone JB and rushes up to tackle the situation.. so he gets shot on the stairs, carries on rushing at Sheila..gets to the top of the stairs, She retreats, fires 3 more rounds into him, he turns back around without being able to lay a finger on her and runs/stumbles back downstairs to meet his brutal end.
It's not impossible and even plausible but I can't see how any reconstruction can prove it