Its not a conspiracy theory its a conspiracy fact. Jeremy's alleged confession does not corroborate the facts of the crime its the same false information police gave AE and RB. Had Jeremy confessed in detail it would match the facts of the crime scene and it does not.
MM went around telling people he was a hitman and RB speculates JB had an accomplice. My theory is since MM and JB were friends police and RB joined the dots (In their minds) That's far more credible than your theory that JB implicated himself and MM because he slept with Sue! That sounds like a storyline out of Eastenders
Of course his confession to Julie doesn't corroborate the facts, he's not stupid. He told her some truths and some lies.
They weren't friends and there is no credible reason why RB would know anything about MM because he and Jeremy were related only in so much as they shared a similar habit. I doubt he'd be telling RB about his latest dope deals and MM and RB didn't mix in the same circles. Your theory isn't credible at all. Also for all we know, Julie might have mentioned to AE that Sheila was found on the bed, not the other way around. By the way, I don't watch Eastenders, it's shite!