The motives of a killer are unique to them but you're getting the idea as far as Sheila is concerned.
I'm pretty sure the the killer's motives were to send her children to the other side, which she did by shooting them in the face while they slept. If Sheila had survived, I expect that her legal team would have successfully argued for some kind of diminished responsibility. With regards to the nitty gritty of her motives, that would have taken getting inside Sheila's head in the run up to the incident and during the incident. Sheila would also need to be able to recall those raging delusions herself, something which she may have refused to do or been unable to do, had she survived.
It serves no purpose for Jeremy to move Nevill. It doesn't implicate Sheila because she couldn't have moved Nevill herself. Therefore, it can form no part of some imagined blame game on Sheila by Jeremy. I don't know whether Nevill was moved or toppled, or his position was some how accidental according to the laws of physics.. but at least I
want to know.
But you're convinced of the police finding reason. Mmm?
What makes you think they didn't? It was the police who had to force entry and the police who then managed the scene right up until the bodies were removed.