I totally concur with your first paragraph. "They had nothing on Jeremy" because Jeremy, by dint of alleged phone call allegedly implicating Sheila and being seen to arrive at WHF AFTER the police, had extricated himself from having at the scene since supper the previous evening.
You are also correct in saying Julie never told police that Jeremy had killed his family. As she was in London she wasn't there that night. Jeremy told Julie that he planned to. As Julie was never on trial, I don't feel it necessary to have her evidence in the public domain. I'll hazard a guess that if you thought Jeremy was guilty, you wouldn't be as interested in what she said. I wonder WHY it "has" to be a lie that Jeremy blamed a hit man?
I would still question what JM had said in her statements whether I thought JB guilty or not,as I'm not on my own in saying that there was corroboration between her and the relatives,according to the scant notes in her new found " diary " which bear a similarity to what RWB had said.
Because JM had mentioned the hitman and got it wrong,the blame went to JB for telling her,which was probably in jest knowing his type of humour which she was seemingly used to,except when he was on the stand,then it was a different matter.