If Jeremy was masked, Nevill would probably still recognise him. I accept that we're dealing with an extraordinary situation, and Nevill may have just fled instinctively and without thinking at all. I find it improbable, though. If Nevill gets hit on the head with the rifle, then Nevill stays upstairs and he dies there.
As for Sheila, my scenario for her as the killer again has Nevill downstairs. Quite simply, I see no evidence for the proposition that Nevill was upstairs that night. Putting him downstairs solves a number of problems.
Just to continue this line of thought, in regard to the question of a 'struggle', I'm interpreting the scene through the likely perpetrator.
In both scenarios, I have Nevill shot on the stairs with the perpetrator above him. Nevill then runs back to the kitchen and the perpetrator follows.
Then:
- if Jeremy is the perpetrator, I doubt there was a struggle. Jeremy is strong enough to negate Nevill quickly and can simply shoot him and bludgeon him;
- if Sheila is the perpetrator, I allow that there could have been a struggle when they got back to the kitchen.
I don't pretend these scenarios are not without problems and flaws. Where is the blood from Nevill en route to the kitchen? Why doesn't Nevill out-run the killer and attempt to flee the house for one of the nearby cottages, or even go in the den? These are similar to the problems with Adam's scenario, but they are of lesser significance as we don't have to explain Nevill passing Jeremy in the tight enclosed space of the bedroom and main landing while leaving no blood, and the related problem of how Nevill manages to reach even the bottom of the stairs without Jeremy catching him, never mind the kitchen.
If, instead, we accept a scenario in which Nevill is downstairs, the problems can be resolved: Jeremy catches Nevill in the kitchen and Nevill is already severely injured and confused, etc. In the case of Sheila, Nevill may have decided to disarm her first: a mistake, but understandable as it's his daughter.
