Scenario 1:
Here we have a young woman with paranoid schizophrenia and a history of violence and threats of violence, who is estranged from her husband and works in dead-end jobs, dislikes her mentally-ill mother who adopted her and has disturbing psychotic delusions about her own twin sons. She is disappointed by a meeting with her biological mother, who returns to Canada. She is also a recreational drug user. Her psychotropic dosage has been reduced drastically in the last month or two. She is already in the house and has access to a rifle and a loaded magazine, with more ammunition nearby. She argues with her father and she starts threatening to go upstairs with the rifle. He cannot lay his hands on her and is also concerned to keep her downstairs and away from her mother and her own sons, so he tries to calm her down in the kitchen and also rings his son (her brother) while she is present. When the son answers, she runs upstairs and starts shooting. She kills her family, all at close range, including her sons, then turns the gun on herself. She is duly found with the rifle on or by her body. She is forensically clean, but it is believed she washed herself prior to suicide, as is fairly common.
Scenario 2:
Now we have a young man with no history of violence, who lives 2/3 miles away. We say that he goes out in the middle of the night to the house, enters and leaves undetected and without leaving any blood traces. He proceeds to kill his entire family, including two little boys in their beds, simply so he can have lots of money now and drink champagne and have meals at restaurants and go to St Tropez. He may also buy a smallholding in Dorset - he hasn't decided yet, let's see what happens with probate. He does this even though he already has a lot of money and a secure future with a large inheritance down the line. He also tells his girlfriend what he is planning to do and then reveals to her what has happened after he does the deed, albeit obliquely in the form of a made-up story about a hitman - in effect, he is confessing to her. She spills the beans to a friend, and this friend engages in horseplay with this mass murderer at her 21st. birthday party a few days later. Like you do. He'd planned it all out and even staged a few phone calls to put the police off the trail. The police fall for it. What can you do, eh? Except for one detective, who drives a classic car and shares a passing resemblance to Inspector Morse. Anyway, our killer spends much of his inheritance before he receives it, splashing out like there's no tomorrow. He also allows the relatives, whom he does not see eye-to-eye with, keys to the crime scene. This, after the police had offered the keys to him, which would have allowed him to easily dispose of any further incriminating evidence missed by the police. The relatives come forward with the evidence instead, which had been mysteriously overlooked by the police themselves. These same relatives stand to gain if he is convicted and imprisoned. He even dropped hints to one or two people of his murderous intentions beforehand, including a hostile uncle.
Honestly, which of these two scenarios is the more plausible?
In my view, one of these scenarios is simple and straight-forward and makes sense, the other is far-fetched and quite incredible, and frankly sounds like the script for one of those B-movies that gets broadcast late at night on Channel 5, normally starring Shannon Tweed and that bloke whose name I always forget but he's always in films like that.
But for a few elements in each scenario that I disagree with, you outlined succinctly exactly why Jeremy thought he could get away with it.
You outline perfectly why it would look bad for Sheila, although the finer points about her meds wouldn't have been known.
What I can't get to grips with is Nevill phoning Jeremy, hanging on -for God knows how long- and allowing her to leave the room, and giving her enough time to run along the hallway, up the stairs, fire accurately enough to kill the boys, leave their room, go into the master bedroom and shoot her mother, BEFORE he catches up with her. Surely, if she'd with him in the kitchen, she'd have shot him dead whilst he was on the phone, indeed, when he picked it up, rather than let him call for help.
You outline perfectly how Jeremy would have seen himself as being viewed through others' eyes ie, there weren't enough negatives, in his eyes, for anyone to suspect him. The weight of suspicion would definitely fall on Sheila..................until, or unless one looks beneath the surface.