The idea of Harry being paroled (not pardoned) for murder seems inconsistent to the way Molly’s trial was handled.
I got the impression during the conversation between Harry and the Merlin when the Korean Kid got the chop that they had changed the process since Harry was tried... In other words, no trial, soul gaze and that was it... Why? Not enough full wizards to go around to train talented kids, they go astray quickly... Even less full wizards willing to take responsibility under the Doom for a kid on the boarder line or not so far along he/she cannot be pulled back... The prejudice of the moment is better safe than sorry because a warlock can do so much damage, so the chop... Molly was treated differently because Harry demanded it, she got a hearing, the hood was taken off her head, it was conducted in English... Harry was as good an advocate as the Merlin was...
I think you are being too simplistic about the verdict on Harry, I don't think it was so black and white... Yes, Harry used magic to kill Justin, but in self defense... Whether or not that is considered black magic or not clear, though Harry did admit in Death Masks that he had used it [against Justin? Not clear..] It may be that any magic used to kill another is considered dark, that is why wardens use a sword to execute.. So yes, killed Justin with magic, ergo warlock and should get the chop.. However, it was self defense against an experienced retired warden, ergo very powerful kid who may have been justified to do what he did and according to his soul gaze, not unredeemable.. Deciding factor? The Blackstaff was willing to take responsibility for him...
If DuMorne were seen as a “good guy” why would Harry not be a executed?
I don't think that part was ever made clear one way or the other... The passage I quoted from Summer Knight implies that there are some who either thought or refused to believe that Justin had gone bad.. There are others who believe that he could have.. It is all twisted up with politics and some refusing that a former warden could have gone bad, verses those who thought Justin was a son of a bitch as Eb called him..