Reasons why the fake random stranger is a bad idea:
Jeremy could not insinuate Sheila. Something he has been doing for 30 years.
The relatives would still sniff around.
Jeremy would still be first on the scene anyway. Threads created.
I have never heard of such an incident. A stranger going to a remote farm house. Killing men women and children. Then leaving.
A psychopath is more likely to attack an individual. Probably one person he could over power. Rather than put himself up against five people.
If the police did not believe the random stranger theory, there would only be one suspect.
A psychopath is more likely to attack in open land and an environment he knows. Not enclose himself in unfamiliar surroundings.
Jeremy could not claim Neville phoned.
The relatives would still suspect Jeremy. And he would still tell Julie.
The psychopath would have to find a quiet way into WHF. Something Jeremy liked to keep secret from the police.
Julie never said this was an option Jeremy considered. Even he is not that stupid.
He could not create the siege and 'Crazy Sheila' situation.
There would be no sightings of mysterious men in the area. As there were not any.
Psychopaths usually attack women for sexual gratification, such as Robert Napper. I have never heard of one killing children and men as well in one spree.
The random stranger would have to attack using a weapon he had brought over. Such as a knife. He would not know about the guns in WHF. Or whether they were loaded.