I quite like QC's scenario Cc, though I myself think the first shot was a accident Sheila waving the gun around not realising it was loaded hitting Nevil in the arm? and there is also room for Roch's theory as well.
The guilters have to be honest and explain why Nevil ran past the shooter in the bedroom and left his family to their fate? He would never do this the fight would have been upstairs. Also how does the shooter get Sheila to cooperate to take the shot knowing her children are next or already shot?
If the shooter was Sheila he would have tried and easily disarmed her, but because it was Bamber and he’d taken four shots his only option was to go downstairs probably to where the other guns were kept or to the phone?
You say you think the first shot was an accident, if he’d only took an accidental shot to the arm he would have took the rifle off Sheila, what about the other seven and the over kill?