Well you only need to read Sheila's Psychiatrist report to see she thought her children/family members were Possessed by the Devil.
Is that not a 'credible explanation' for the morning of the 7th with Sheila's history on record.?
I mean when did Nevill phone Jeremy & the police ?
When did Sheila start shooting ?
What was Sheila doing when Nevill made his phone calls ?
How did things escalate between Nevill and Sheila ?
Where was Nevill when Sheila opened fire ?
Who did Sheila open fire on ?
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Everyone knows the crime scene, body and bullet locations. So it should not be hard.
I have created a Bamber scenario which matches the crime scene.