Going back to the phones.
It is much more likely that Jeremy would attack Neville in the kitchen. Both phones were in the kitchen. Jeremy had to stop Neville making a phone call & identifying him.
If Sheila was planning to commit suicide, there is no reason to stop Neville making a call. By the time the police arrive, everyone would be dead.
That being the case,Jeremy didn't,or hadn't bothered to do any wiping,etc after himself. The table where the phone was with someones bloodied fingers,,the rifle,,the silencer. He did NOT retrace his steps,.,so he wasn't the murderer,as murderers always leave clues behind,no matter what. Even a droplet of sweat,fibres from clothing,all manner of clues,,but nothing.
To make matters worse anyway,no time of death was ever recorded,,no scrapings from under the fingernails taken and countless other unrecorded omissions.