I fully understand the arguments QC makes. However I believe there is another option he has not set out.
There is a possibility that during the course of an incident the police make a serious maybe unforgivable mistake. They seek to conceal this error and close down the case to their satisfaction.
Later other actors who have 'issues' with this outcome deduce or are told by someone (whistle blower) what took place in fact.
They then pressurise the police to reinvestigate the crime who then pursue an innocent individual who has a very close connection to the crime. They frame a convenient 'patsy' in order that their initial mistake and cover up remains hidden.
In the way you've described it, that is option (i) - which is framing an innocent man. You're just proposing a motive for it, which is that they've messed something up and don't want anybody to find out, so they give in to blackmail from third parties in contact with a covert whistle blower. In law, what you are describing would be a scheme of blackmail under section 21 of the Theft Act, a very serious offence. That's in addition to perverting the course of justice and perjury. How many people were involved in this and what did each party know and not know? Did the core conspirators rely on a belief that the case against Jeremy was weak and he would be acquitted? Why didn't the police just arrest the blackmailers and cover it all up?
The basic problem with that you propose is parsimony: it's easier for the police to cover up and deny the mistake itself than to give in to blackmail from third parties and concoct the elaborate framing of a probable innocent man. One is simple, low risk and morally-defensible even if dubious, the other is complex, risky and morally-outrageous, in fact evil. Most people would believe the police and the authorities would tend to back up the police - especially back in the 1980s, when external oversight of investigations was rare.
Essex Police would just arrest the blackmailers and say they were either lying about the mistake or exaggerating it, and most people - including the courts - would accept this. If Essex Police shot Sheila, they would just admit it, discipline the officers involved, arrest and charge the blackmailers, hold an inquiry, apologise, and move on.
In other words, I find the motive unconvincing, not because what you describe is unlikely - quite the contrary, I imagine convert whistleblowing about serious mistakes is commonplace - rather because it is unlikely this would form a motive for police officers to frame somebody who they know or believe is innocent.
