I don't think Jeremy phoned Julie before the murders - he may have called her before the police because he was a bit of a wimp and could not decide what to do.
This was supposed to be the most clever planned murder - so to me the most strange thing is that he would tell Julie. I just have this nagging feeling that the police actually contacted her to try and get information. And I know there is no evidence - but something does not add up.
Your judgement is skewed and way off. You are so biased you even ignore the solid evidence that Jeremy called before the murders.
Julie's roomates confirm he called and more importantly so does Jeremy. The fact Jeremy called her is proof that he went home and didn't simply stay there.
He called her prior to the murders, right after the murders and then again around 6AM to tell her not to go to work because he needed her to speak to police to help substantiate he was worried about his family because he received a call from Nevill.
Jeremy figured she loved him and would not rat him out. Thus he told her how he wanted to kill them and kept her appraised of some of the actual details. He even thought she wanted to help him by giving him sleeping pills. He was right about her not ratting him out at first but when police came to her because a friend told them what she said then she was stuck between telling them the truth or risking obstruction charges.
Police did have evidence the case could have been made without her testimony.