Probably a further escalation of events.
Forgive any slowness on my part - but why would she need to 'fit everything' in to that time-frame?
Neville wouldn't call the police if Sheila had started shooting people. It's too late.
So in 22 minutes Sheila -
Fired 24 bullets. Into 3 sleeping people and one awake person. In three different locations. Both up and down stairs.
Chambered twice.
Reloaded twice.
Fought and brutally beat Neville.
Postioned Neville on top of a coal scuttle.
Burnt Neville's back.
Wrote a long suicide note.
Had a shower.
Put a nightie on.
Fetched a bible.
Read a bible.
Shot herself once.
Went downstairs and put the silencer away.
Shot herself again.