The shots to the children - surely the MOST emotive for a Mother - were the MOST controlled.
Could Sheila have calmly put a shot in the back of the head, moving closer as that 'arc' was created? Then repeated it with the second child?
According to the family, Sheila had bad co-ordination. Would mania have improved this? Plus she didn't know one end of a gun from another, having never been known to shoot in her adult life.
The clinical nature of the children's death doesn't tie in with a mother on the rampage, trying to rid her children of the devil (her earlier preoccupation).
Yes, on paper, any of us are capable, but...