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.