In several episodes we see Columbo utilizing "outside" help to bring in the killer. But how many of those people might be reluctant to get on the witness stand but then Mark I can think of 2 off hand.

Dominic Doyle. My guess is that his contribution will only go so far as he's probably had more than enough dealings with the law. I just can't see him getting up in a courtroom.

Arte Jessup. Keep in mind that by his own admission Arte likes life on the inside. I wonder what his old lady would think if he had to go to court. I just don't see him getting up there in front of the judge and the jury and other people and telling them how it was. And then he's going to get arrested anyway for the other robberies did he did prior to the actual murder. Which he wasn't involved in.

I'm sure there's a lot more, you guys take this and run with it

I think Arte would be more than happy under the circumstances given who the real killer is, and the fact that he tried to frame Arte as well. Putting the Commissioner behind bars would be a badge of honour for Arte with his fellow criminal colleagues.

There's at least one story where the subject comes up. In Lady In Waiting, Columbo tells Beth that "He won't LIKE it," but Peter will testify if he has to.

Even though one of the the victims is her uncle, Janie wouldn't want to testify against Ruth Lytton.
(Assuming she would know anything that could really be used.)

Somewhat similar to Peter Hamilton, I think that Karen Fielding would fit into the same category. While she really doesn't want to testify against Adrian, I'm sure that she would bring herself to do so.

I'd love to be a fly on the courtroom wall when the Devlin case ("The Conspirators") goes to trial. I can just imagine the backstabbing and emotional testimony we'd get from Poet Joe, George ("you've got such an executive flair about you") O'Connell, Kate ("the majesty of Queens") O'Connell and young Kerry ("late of the Emerald Isle") Malone as they take the stand, trying to protect and sell out all at the same time. If Joe doesn't have a drink screwed into his fist, he might be able to keep his wits about him.