You're all mixing things up chronologically so let me elucidate. The plot to kill five people was Jeremy's and Jeremy's alone and whether by Julie's acquiescence as some maintain Jeremy was buttressed in his determination is something Julie will have to live with for the rest of her life.
At one of the few meetings they had alone together at Blazer's restaurant,Blackheath after the murders Jeremy told her "not to blame yourself..I would have gone ahead with it anyway" and one wonders just when the following occasion would have been when all five family members were together under one roof just as they had planned to be at Christmas 1984,when Jeremy's preferred method of killing on that occasion was arson and not shooting,as related by independent witness Charles Marsden.
As far as the aftermath of the murders was concerned,Jeremy's plan had been successfully accomplished,he had fooled no less a man than the head of the investigation DCI Taff Jones,and it was evidently Jeremy's intention to play for time when ineluctably the murders however horrific would gradually fade from public memory.
