No, the job of the Knights is explicitly to save the Denarians--stopping them from hurting people is what they do when they fail to save them.
No..... If this were the case killing them would not be an option, and it is. Michael wasn't out there searching for Denarian's to save. He went out there by TWG's directive and fought baddies of all kinds. When he faced Denarian's he gave them the option to repent.
If the job of the Knight was to save the Denarian's he would have been out a lot more trying to save them when in fact the majority of the time he wasn't even fighting them but other things. Outsiders, vampires, demon's, ghosts, and a Dragon are just some examples of his missions.
Also, they picked up the coin by their own choices, so we know that "they made a choice" is not necessarily going to release the Knights from their responsibility to help them.
Again, they don't seek them out to help them. I don't recall a single time that a Knight's only objective was to locate a Denarian for the sole purpose of saving them. In every case I can remember it was because the Denarian was doing something bad, and the Knight showed up to #1 stop them from doing it, #2 save the person if possible. The first objective has always been to stop the Denarian.
I see no reason why they would show up to rescue a Denarian from Demonreach if they screwed up bad enough to end up there. Unless the person repented, and TWG knew it and wanted them to remove the Coin from the prisoner. Other than that a Knight isn't going to care if the Denarian is locked up.