I can only think, without having any specifics laid out, that the only thing that doesn't add up is motive.
It's not the jury's responsibility to find motive, or even to wonder whether he presents as a murderer.
I would like to know what the judge said about the defence introducing accidental death into their closing speech. He shouldn't have ignored it.
I agree with everything you've said, Tortoise, and wondered if the implied/open rebuke to defence counsel meant it wasn't said in open court. But then how would it reach the jury?
There were so many other less fundamental bits I'd have loved Judge B to recap in the excellent way he did with the ludicrous 'Are you GGHW?' exchange. I hope the jury recall the bizarrely detached replies IS gave so often, for example - was he angry when he realised Joe and Nick had dumped Helen's body in the cesspit, no he was 'annoyed'.
ETA In fact 'angry' was an odd word too, though I assume Trimmer chose it deliberately. You might have expected 'Not angry, no, distraught, desperate!'