How would he know in advance that a medical professional would not take temperatures at the scene? How could he know in advance, that a medical professional would not establish times of death for victims, that did not fit with timeline set by his own claims?
How can he ensure that a struggle with Nevill or Sheila (a mother with fingernails, potentially fighting for her children's lives) or that furniture being bumped in to, would not cause him to have visible marks on his own person?
If he can't guarantee that he will not be marked during the killing of five people, in a confined space with furniture, what is his plan B to explain the marks?
He turns up to meet the police with a black eye or scratches and claims 'oh yeah.. that's just a coincidence.. nothing to do with anything that happened here'.
It's laughable Lookout.
Roch, I'm convinced that there are certain things which it would never have occurred to him to think about. It's only with hindsight and MUCH more knowledge of the psychology behind the thinking of those who commit crimes, that we can ask the questions we're asking now.
On top of that, whilst he may have written the script, he had no way of knowing how the victims would play it out. He didn't have that much control, other than in his mind. He didn't bank on his father putting up a fight. It would all have been so much easier if they'd all, other than Sheila died in their beds, like the twins, without moving from them. Sheila, of course, had to be placed as near as dammit, to look as if she'd done it. He got it wrong because neither she -nor any other loving mother- would have chosen to die apart from the children they'd killed, in favour of a person who'd given them grief most of their life.