In a nutshell, basically that's what it would mean for Jeremy to be innocent. The cast involved would be massive in order to maintain 'the secret' and also many police officers would have independently decided to lie, withhold information and frame Jeremy even before the scene was discovered. It would be too far fetched even for a work of fiction and we're being asked to believe it actually happened.
No, it wouldn't...
I myself have been on the receiving end of corruption and dishonesty by a few bad apple cops. It does not involve everybody else being in on it, as you are saying. All it takes is one bent cop, or one group of bent cops, and everybody else who becomes part of the prosecutions case is carried along on the back of the fabrication, or that dishonesty. What you end up with, is the sniggering bad apple cop, or group of cops, chuckling amongst themselves, because all the other cops, and prosecution witnesses really believe that their contribution to the case is 100% genuine, and truthful. I can provide many examples in my own cases spanning six years when I was considered to be a low life career criminal, but instead I will talk in terms of J's case. OK, let's assume, that David Boutflour found a silencer at the scene on Saturday, 10th August, 1985, but instead of Peter Eaton handing it over to DS 'Stan' Jones on the evening of Monday, 12th August, 1985, Boutflour retains possession of it, until Jeremy gets arrested on the first occasion, 7th September, 1985, or there about. Let's assume, that once J is in custody being interviewed by cops, that David and the other relatives have been talking amongst themselves, fuelled by Robert Boutflours hatred of J. What happens is, David gets his sister, 'Annie' to hand over various exhibits that David had removed from the cupboard in the den at the farmhouse on the previous month ( let's say, 10th August, 1985). So amongst these items, is the silencer that David had kept possession of for over a month, before he gets his sister to hand everything of possible interest to DC Oakey, on the 11th September, 1985. Putting those facts to one side for the moment, imagine that prior to David arranging for his sister, 'Annie' to hand the silencer, and the other stuff over to DC Oakey , that the relatives were aware that J would be appearing in court in a day or so, charged with the burglary at Osea Road, Camp Site, an offence which involved Julie Mugford, as his 'accomplice', and that J would be applying for bail, and was requesting sureties. All the relatives 'turned him down', they weren't about to help J get bail. And, at around this general time, there was a specific conversation that I would like to introduce you all to, between Annie and David. It went along the lines that they weren't going to help him get bail, because they wanted him locked up. It was 'then' that David said to Annie, ' don't worry, I've got something up my sleeve'. I kid you not, this is what David said to his sister. How do I know this? Well, I know this because Annie was always jotting down notes on pieces of card, and guess what? She recorded what her brother had said to her on 'that' occasion. So, what was it, what was 'this something' that David Boutflour had up his sleeve, which would help put or 'keep J locked away in prison' behind bars?
On an occasion after Annie handed over to DC Oakey, the silencer, and everything else, on the 11th September, 1985, David contacts the police, to tell them that 'he has found the silencer to the gun'...