In a scenario in which Sheila was upstairs already when Nevill confronted her, there are problems.
She had already killed the children and shot June at least once, and Nevill tried to stop her killing herself, is that right?
In that scenario, when does Nevill make the phone call to Jeremy and why?
If Nevill makes the call after discovering or realising that the children have been shot, why does he ring Jeremy at all? Why not 999?
How do Nevill and Sheila end up downstairs and how does the struggle or fight take place?
Nevill could have been trying to stop Sheila killing herself (before or after she killed her children). But Nevill trying to stop her committing suicide also raises a problem. It's not plausible due to the specification of the weapon. Just think about how she would have to go about killing herself using that rifle. It would be a very deliberative process, giving Nevill ample chance to intervene and stop her.