He got a full confession out of him. And shortly after the interview he was seen chasing after Kaiser Sose. This is the definition of good investigator.

yeah, but kujan seemingly has tunnel vision, and wasn't willing to accept any possibility regarding who soze was at all. And yes the very evidenced added up but damn, he never really checks his Q's, etc.
If you want to be taken seriously, you have to be a serious person.

Real-life detectives often develop an unwavering fixation and certainity of a specific individual's guilt.

Sure, but Kujan wasn't even officially on the case. Rabin was and he just had to go aong with the DA.