Why didn’t Sheila shoot Nevill while he was downstairs first on the phone and if he was too scared to confront her in the kitchen waving a rifle about and he had to phone Jeremy for help.
Because that is simply is not how the events transpired. Sheila wanted to shoot to June, when Nevil came upstairs to help he became a target. This was a stochastic commotion not something though out rationally.
why did he run upstairs without his gun/rifle or some implements to confront her.
I don't think shooting your own daughter would cross anyone's mind in that situation.