It's not credible for so many officers of such differing levels of experience, grades and roles to be incredibly stupid concurrently.
The reason why police went with Sheila is because they knew what had happened in the farmhouse. That being so, the evidence of what had happened caused a reviewing officer to reach the same conclusion approx one month later.
The reason why Mugford gave evidence against Bamber was because she found her self in the position of being a pawn, with only one way to move out of the danger threatening to engulf her.
The reason why some senior EP officers turned on Bamber is because they were corrupt, within the reach and influence of Robert Boutflour and under pressure of having to face something become exposed, namely the events surrounding the TFG operation and it's aftermath.
The reason why the relatives went for Jeremy Bamber is because they knew nothing of note regarding Sheila's condition, her episodes or her psychology. And because they regarded him as a threat to their own livelihoods. He became in effect, not 'one of them' (at least if he ever had been, he certainly wasn't after 7th Aug).
The reason why Jeremy Bamber became the scapegoat was because the only people able or willing to protect him from such malicious actions against his person were both killed by his sister on 7th Aug. He had no protection from Robert Boutflour. It has to be said also, that arguably, he lent him self to being framed by having committed petty crime and in the eyes of some, displaying arrogance and a lack of grief.
What springs to mind here is that the reviewing officer was in all likelihood, not prepared to become corrupted, as was being expected of him. He failed to come up with the goods that had been requested by ACC Simpson, Robert Boutflour and their lackeys (for example Mick Ainsley, the ubiquitous Stan Jones and Bob Miller).
Two sets of EP officers. One set outside the reach of Robert Boutflour. One set within the reach of Robert Boutflour.