At this moment in time I'm trying to work out a plausible scenario of the crime from beginning to end. I think it's the responsibility of everyone posting here to do the same regardless of who they think did the killings, not maybe this or maybe that. I think that if people are going to refer to a pre-laid plan, they had better flesh it out and stand by it or lose integrity.
As it stands, with the information I've got, I think Sheila committed the crime. The theory repeatedly asserted in CAL's pro-guilt book is that Sheila had no knowledge of guns; this is only partly true since a child can load the weapon and fire it, in the U.S. they most certainly do with this type of gun. What Sheila didn't know was how completely inadequate the weapon and ammunition was for killing adult human beings, while Jeremy would have known with his experience of guns. What we have here is not "overkill" but underkill.
Sheila would have discovered this as the shots that she thought would kill outright were only resulting in cripplings and maimings, that's why I think she revisited each body and fired a coup de grace when they were all in no position to fight back. From that point onwards Sheila would have been utterly demented, she wanted to die, and perhaps needed to die, but she had a gun that was was not guaranteed to kill her if she turned the gun on herself. She could have ended up with crippling brain injuries or paralysis, not the instant deliverance to God and her twins as she wanted.
She was alive when the cops broke in.