Hold on, wasn't the flak he got for the case before the show started actually because he was covering for his wife (who was also working on the case)? She was having an affair and left the evidence in her car where it was stolen. He didn't say anything so their daughter wouldn't know, and took the fall for the case's failure, even though it wasn't his fault at all.

I didn't quite get the same read on the show you did. It seemed like the dynamic was that Olivia Coleman couldn't imagine anyone she knew being the killer, contrasted against Tennant being aggressively willing to suspect anyone, which is how they were able to rule the various suspects out.

It's admittedly been years since I saw it; I don't remember the entire mitigating bit about covering for his wife, but a lot went on in that series finale and I've had covid a few times since.

I like your read on their dynamic as foils to each other; I'll have to give it another watch with your read in mind.