So in effect what you are saying is that Ralph received most of his shots in the bedroom and then ran or staggered downstairs? So using your same scenario if Sheila received her non fatal wound downstairs would she have been able to run or stagger upstairs with just one wound?
The last time I checked 4 is = 4 and 4/8 is half not most.
According to the medical examiner the 4 wounds to the left of his body that were suffered upstairs would not have prevented him from going downstairs the kitchen. If he would not have been able to do so then the struggle in the kitchen would never have occurred.
A piece of his lip was shot off, he had a bullet in his shoulder, a graze of which part of the fragment ended up in his side and a shot to his jaw that ended up severing his voicebox. None of the se wouldns would have immobilized him right away. None of these wound would have been possible to deliver in the position his body was found in.
How would he go down the stairs so that his side profile would be directly facing a shooter at the top of the stairs? Who walks down stairs sideways? Even on purpose it would b difficult. Someone being chased by a killer would be running and that is if Nevill was running away. For all we know the killer was running and Nevill was giving chase. Nevill tried to wrestle the gun away in the kitchen so it could be either way. That is where either where he caught up with the killer or the killer caught up with him and whichever the case is he then tried to disarm the killer.
For sure the gun was out of ammo or Nevill would not have made it out of the bedroom. But for the gun being empty the killer would not have fled to the kitchen with Nevill giving chase or alternatively manage to get hrough the killer out the door to run to the kitchen with the killer giving chase.
2nd point. So now you are trying to convince us that Sheila could not have since all his wounds were of a downward trajectory and that she was too short. But Jeremy although still too short to have shot Ralph from that angle could have exagerated his height? I see so sheila is too short to have done it and Jeremy is too short to have done it. But Jeremy alone could have exagerated his height? 
Explain how Sheila could have fired the shot into Nevill's jaw or shoulder in a downward trajectory unless he were in a lower position viv a vis her. His jaw and the area of his shoulder are higher than where she would hold a gun level. A level shot is relatively straight. Since these locations are higher than a level shot that means if she hit such locations and he were standing and she were standing on the same level as him then the shots would be upwards. What is confusing about this? It is not rocket science.
So other than if he was sitting or she were standing on something, explain how such could occur.