So a stroke of luck then for Mick, Stan, Ann, David and Bobby, that Fletcher's innocent mistakes just happened to dovetail neatly with the prosecution case.
Well there are two explanations for that:
Explanation 1: Jeremy is guilty and the evidence broadly reflects it.
Explanation 2: As you say, Fletcher's innocent or negligent mistakes and errors just happened to dovetail neatly with the prosecution case (if you want to interpret it as favouring the prosecution case - as I explain below, you don't have to).
The reason for Explanation 2 would be that Fletcher, being human as well as an expert, will be guided in his work by the dominant narrative given to him by the police. It's rare that a scientist will think against it unless he finds something that incontrovertibly goes against what the police hypothesise.
The police may say: "We think the culprit is this person and we think this is what happened and he did it this way. Let us know what you find."
It's no surprise that he then finds something that happens to dovetail with the police narrative. It's what you'd expect, as long as there is something to find.
But what he found is open to interpretation. As I've explained, the pull-through test was never reliable, but if you accept the result at face value, you could interpret it in Jeremy's favour (subject to further expert evidence from somebody who knows about fluid dynamics/tension dynamics as it applies to blood and ballistics).
One way to prevent dominant biases is to ensure that there are two different scientists/experts examining the same evidence, so that there is somebody available to challenge whatever is the dominant assumption.