Did they have x-ray eyes to see beneath his clothes? Julie certainly shared his bed but she implied to the police that she hadn't felt able to have sex with him after the murders which suggests she may not have been too keen to examine his body closely. What makes you think the police would have been able to see any injuries on him whilst the were taking tea with him? Had there been any, he'd undoubtedly have gone to great lengths to cover them -didn't a glove come off at some point during the struggle with his father?- and they'd have passed off any need for layers of clothes as shock.
Why do you think Julie did not cover the vital bit of information about what he did with his clothes ? Its strange that all her information fitted the crime as known by the police , but not this vital bit of information.
If it was Jeremy I don't think he would have risked the clothes being in the house just in case he became a suspect immediately - especially after the two shot fiasco. He must have had another plan.