I agree that you have to be the one to make the choice to do the right thing; what I mean by saying that the Knights' job is saving souls is the whole thing in "The Warrior," where Harry basically goes around doing things that end up helping people to do the right thing and improve their lives, even though he doesn't realize it until Uriel points it out. I figure the Knights probably have more awareness of what they're doing, but otherwise...Uriel explicitly describes people doing this as being "warriors for the light" or something--it seems like exactly what the Knights' job is, only they're more focussed on Denarians rather than innocent kids (probably because there are lots of people who would help innocent kids, and not many who would help Denarians and such).

Harry unknowingly helped others to make the "right" choice..  His actions created the climate where the right thing could be done...  Though he did prevent Michael from beating to death the guy who would harm his son, that was more direct action..   Knights are a little different,  when confronted with an evil action they'd try to stop it like any good person would, but they do not judge the person who did it, nor do they feel it is their place to punish that person... That is why except for that one night Murphy realized she couldn't be a Knight..  All her life she had been a cop and she used her own judgement to stop bad guys and she saw them or judged them as being bad.. When she judged Nic with a "Damn you,"  after he had surrendered and tried to execute him, she broke the rules and the Sword.    Like Murphy, Harry has no problem in punishing those he judges as bad or criminal, he also often repeats that he is no Knight.  After the foolishness with Lea has never tried to use a Holy Sword again. Though one wonders if he would have if pushed on the island in Small Favor.

No. Not to the Knights. The Knights' explicit job is saving souls. If they were willing to prioritize preventing the Denarians' acts of mass destruction over that, they wouldn't have refused to touch Cassius after he surrendered his coin.

No,  once Cassius gave up his coin he became a free mortal once again..  The Knights do not save souls they give souls enthralled by the Fallen of the coins a chance at free will once more...  From the point where he surrenders the coin until his death, what he decides to do with what remains of his life is up to Cassius...  If he chooses to atone and seek redemption, well and good... If he doesn't, well it is his choice and no longer the concern of the Knights.

Though of course, Michael has a vested interesting in his friend Harry not being a mass murderer.

True, but I really doubt if he believed it were true he'd let him off the hook.

I believe it was argued, or Michael argued that those kids that burned up were already dead.. Poisoned or fed until death by the Vamps, they had already culled out and took away with them the the new recruits, they were at Bianca's place along with Susan.

Why not? Shiro went to rescue Harry from Nicodemus when he was basically in the same situation as Marcone was--i.e. "take up a coin or else."

In Harry's case in Death Mask it was a little more complicated than that.   The mission from Heaven
was to keep Harry safe,  yes, Shiro unknown at the moment was dying of cancer was willing to take Harry's place.

Isn't that what he did?

Yup, and that is the difficult concept for would be Holy Knights and non-Holy Knights to grasp..  It is the concept that tripped up Murphy...

I'm curious why you say that. There are plenty of common mythical creatures that have barely been mentioned much less shown up in the series, there's no reason for a sphinx to be here more than a griffin, harpy, gorgon, hydra, basilisk, manticore, cyclops, etc.

Yeah,  I don't think we will see those creatures either..  I think it is the first part of the poem.. The White Council falling apart,  more warlocks because not enough wizards willing to train future wizards..  Disruption with in the White Council, the Winter and Summer Courts by Nemesis,  rot from with in..

You keep your tests once the class you took them for is over? Huh. I don't, so I hadn't included that in my analogy. If it bothers you, just pretend that the teacher didn't hand the tests back, and you found out your grade by looking on the school's website.

I would if I felt cheated... 

Nope. If you have serious confidence problems to do with schoolwork, and you explicitly know that you have problems with the specific subject, you're fairly unlikely to go up to the teacher and demand an explanation. Remember, in this analogy I am the equivalent of Harry, who has a guilt complex (equivalent to "confidence problems to do with schoolwork") and knows that he's messed things up (from his perspective, anyway) with Murphy previously (equivalent to knowing that he's struggling with the class material).

One might consider a therapist...  Still, if the teacher in a math class flunks you due to penmanship, there should be a note to the fact on you paper...  Harry has a guilt complex not because he is sloppy with his work, but perhaps maybe he is too good at it..  Thus he takes the blame when he doesn't deserve any..