I don't agree with you that Nevill would go up and down the stairs 5 times and let Sheila hit him with the rifle while fully fit.
Nevill would instantly take the rifle off Sheila while fully fit.
He was 9 inches taller and 8 stone heavier. Also a male & several times stronger.
Sheila was very docile, uncordinated, weak with no weapon experience.
The rifle was a low powered weapon used for shooting vermin.