It does seem that the only reason a fully fit Nevill didn't jump on Sheila, is because she was holding a rifle for shooting vermin.
The evidence & suggestions from other people shows a fully fit Nevill & Sheila were inches away from each other on several occasions. Even when they were not, Nevill could have charged her.
After being shot from close range Nevill chose to run, wrestle & (apparently) give Sheila 28 cuts, gouges & abrasions.
Using his massive physical advantages either pre or post shootings would have been easier, quicker & would have saved everyone's life.
I thought I was leaving, but not quite.
Well, many possible reasons come to mind as to why Nevill would not overpower Sheila.
1. She held a gun on him the whole time.
2. Having confronted Nevill, Sheila told him she had shot/killed June. Nevill goes into deep shock
3. Nevill tries to disarm Sheila, they grapple, she claws his arm away and then she shoots him in the left arm
4. It is jolly painful to get shot and have a broken arm and Nevill finds this rather debilitating
5. Despite being shot, Nevill tried again, and Sheila fired another shot that grazed his chest
6. Sheila told Nevill to sit down. At gun point, he obeys
7. Sheila shot Nevill in the face twice. That caused excruciating pain and heavy bleeding. Cue even more shock
8. Sheila stood very close to Nevill and fired four bullets into his head.
A person holding a semi-automatic rifle has many advantages in that situation?