I don't think he did plan it a year in advance. I think he talked about killing his parents but I don't think there was much planning involved because of all the mistakes he made.
By the way, who do you think he called first?
Based on what he told Julie his planning for most of the time was just in the initial stages. It doesn't look like he put tremendous effort into the fire plan. He never got to the finer details so far as we know and he abandoned that plan when he tried to get to the finer details.
He took what in he eyes of the law are known as steps in furtherance of his plan. Buying the gun was an example. But even after the gun was purchased that doesn't mean he had thought of everything he could with respect to the finer details. But he told the farmhands there is no way he would share things with his sister so it shows he early on decided to kill everyone. The main details he thought up were to shoot everyone and blame it on Sheila.
Obviously that means everyone had to be staying at WHF at the time so it would take a while before the sitatuation was in place for him to implement his plan. That meant he had plenty of time to think about things in the broad sense but didn't have to think about the finer details until close to the event.
Many of the things that screwed him are things he thought up and planned. We have no way to know when he thought up the idea of pretending Nevill phoned him or calling Julie to buttress his acocunt he received a call from Nevill. We only know when he told Julie he planned to do such. Which shows he wasn't just telling Julie he wanted to kill them he actually told her plans so she should have had a clue he was really contemplating it. We don't know when he came up with the idea of pretending he left the rifle and bullets out and decided he would stage the bullets. We don't know whether he planned in advance to take the moderator off or he he just took it off after he realized it would be too long for Sheila to kill herself with it. Putting it away suggest to me he planned it all along because otherwise he could have left it next to her. He didn't want police to know it was used at all because a crazy person planning to kill everyone and committ suicide would not care if anyone wakes up.
Other details were how he would travel there, how to dispose of the clothes he used and things of that sort. How early did he decide to use a bike? We just know when he told Julie he decided to use a bike. When did he decide to use the windows to enter and exit? We can't be sure. So we have no way to know for how long he planned various aspects before telling Julie.
Planning for a long time doesn't mean something is a good idea though. Clearly the call to Julie wasn't a great idea because he had no valid reason for making such call. He didn't plan out the call from Nevill well enough to have a way to prove a call was placed from WHF to Goldhanger. He assumed people would just believe it happened based on him saying so and calling Julie after he claimed he received it.
Thinking a long time about something doesn't mean you will think of everything important or plan correctly. He clearly did not think about things like Sheial should have had backspatter on her clothing, drawback, GSR and he is in good company because it is rare that criminals think about such beyond getting rid of their clothing containing such though they don't even always get a chance to do that or do it even when they have the time. In the Amityville murder case, the son went to work in the same pants he wore during the murders, and stayed in them when he claimed to find the bodies and police were summoned, he only changed his shirt. His pants were dark so he didn't notice the high velocity spatter on them. There are tons of examples of things like that.
What planning did he implement in advance?
1) The overall plan to kill them at night with the house locked up entering and exiting through the windows so it would seem that it had to be someone inside the house that comitted the crimes
2) To use a semiautomatic gun and thus had his father buy one
3) To attribute it to Sheila's mentall illness thus make it look like a murder suicide
4) To kill everyone including his nephews so that he would get the full inheritance
5) To use a bike a took his mother's bike to his place shortly before the murders
6) To use gloves so he would not leave any prints
7) To wash up and change his clothing after
To call police and say he received a call from Nevill that Sheila had gone crazy and had the murder weapon in her hands
9) To use the mdoerator so no one in other rooms would wake up and make his life more difficult.
We don't know much about how detailed beyond his he thought about it. The most important part of his plan was that everyone needed to be at WHF to pull it off so that determined when he could do it. That explains why it took so long before he could do it, not that he was waiting in order to do meticulous planning.
Truly meticulous planning would have featured thinking up how to plant GSR and blood evidence from the victims on Sheila in a way that would not reveal it had been planted. Planning for a year doesn't mean you have the foresight to think of such or skill to think of a way to do it. It is folly to think that had he thought about it long enough his plan would have been flawless, he picked some bad ideas that seemed great to him but he didn't think abou how him calling police implicates him if it came to pass that Sheila didn't do it. His attempts to frame Sheila all backfired including staging too many bullets in the kitchen and his non credible account of leaving the murder weapon out with such bullets. Smart planning requires more than just time, it requires having the requisite knowledge to think up the perfect plan and to consider all contingencies. There is a military saying, "no plan survives first contact intact". First contact is when you first come into contact with enemy forces. Something always happens differently than planned and then you have to stray from the original plan.
We have no way to know what things Jeremy did simply because things did not work out as he originally planned. That would be the more interesitng thing, to hear from him how things changed as the situation was fluid. It is morbid to say that but it is interesting nonetheless.
