If Harry had failed Nic's daughter would not have been sacrificed because the ice test was before the human sacrifice.  Nicodemus would have been upset but Harry Dresden would have been dead (a big plus for Nicodemus), and Mab would have still owed him a favor + interest for her failure to keep her end of the bargain.

   She did keep her end, not her fault that her Knight failed..

I don't think that Mab not telling Harry was a test.  You don't test someone like that over something so serious because it's no longer a test, it's real.  My guess is either she didn't see a reason to tell him, or even him knowing would be worse than not knowing, or part of the deal with Nicodemus meant she could not tell Harry the nature of the artifacts.

You are forgetting what Hades told Harry,  the process of inquiring the artifacts/weapons is a test... As in only the strongest and smartest is good enough to end up with them, this is the safe guard..  There are no cheat sheets,  for the war to come Mab wants her Knight to be the smartest and the brightest and holding the weapons..  Nick thinks he is the smartest and the strongest, so he thinks he is being clever just asking for the Grail, figuring he will then easily get the rest of the artifacts..  Mab is gambling that Harry is the smartest and the strongest, so he will end up with the weapons, and if not, he dies, she looks for another Knight...  Nic loses because if Harry fails he isn't going to get the weapons..  If Nic really was the smartest and the strongest he wouldn't have needed to call in the favor in the first place to use Mab's Knight.

Here's a question .... Why didn't Nick reveal that he wanted more than just the Grail?  He told the group that much, why not say he wanted all of the relics of the Crucifixion?  I know he doesn't want to reveal too much to his enemies, but if he had said he wanted all of them, then Harry would have been obligated give him all of them and not just the Grail.

When he told Harry and crew about what he was wanting, Harry didn't know they were weapons, so it could have been wrapped up in the need to fulfill the obligation to Mab.

Because if he did, the whole mission wouldn't have happened, or that is what he was thinking.   Personally I think it was like a line from "The Lion In Winter..."   Nic knows they were all weapons, Mab knows that they are all weapons, and Nic knows that Mab knows..."  The only one who didn't was Harry, who Mab happened to be testing further in my opinion... Don't be shocked if she is very aware that Harry pocketed the rest of the weapons and that they are just where she wants them...

They're not "entitled" to a trial -- the vast majority of Warlocks are executed in the field by Wardens.

That doesn't mean they weren't entitled to on, just that for the most part, they don't do them anymore for lack of the manpower...  The response when Harry asked for one for Molly was, " I thought we didn't do that anymore..."   If what you say was true, Morgan would have given Harry the chop long ago because until the end he never stopped believing he was a warlock...

   Their responsibilities include protecting both the Senior Council and other White Council members from attacks, dealing with the Vampire Courts, the Faerie, and warlocks and serving as judge, jury and executioner when in the field against violators of the Seven Laws of magic. Many warlocks are killed resisting arrest, or apprehended after their crimes have earned them a death sentences. Hearings before the Senior Council are rare. Dresden Wiki

No, they are not judge and jury!  The accused are entitled to a trial, even if it has evolved to almost a kangaroo type event where the outcome is predestined because of 1] practicality, not enough wizards willing to risk to the Doom to rehab a young would be warlock, 2] because the Merlin in particular believes the results are mixed at best to disastrous if the rehab fails...   So it may have evolved to a point where young offenders are simply killed for a variety of reasons by the Wardens, but that isn't how it is supposed to be..  So these days the Council does a lot of looking the other way unless someone like Harry insists that a trial is held..   The whole point of the trial for the Korean kid in my opinion was an object lesson for Harry by the Merlin that trials are mostly a waste of time..

When Harry tried to bring in Molly for trial the merlins reaction to was something like "why did you not kill her yet" and "you are wasting time". Even with a warden they do not really trust like Harry the overview is minimal.

I am sure a lot of wardens do their jobs as expected but there is a lot of room for abusing power, cutting corners, acting on tunnel vision, ....

And little chance of discovery. In such an organisation you need only a few bad apples to get a reputation among people who do not dare to complain because there is no independent complains department and wardens will cover each other. Because that is what members of closed organisations do if they get the chance.

And if a bad apple is in a position of influence others will copy his behaviour because they think it is normal.

Wardens have power. People behave differently around them which makes them less aware what is going on.

And most importantly pressure to get results can do the rest. The whole structure promotes abuse of power and no real brakes on it. The only brakes are the wardens own consciences. That is not enough.

So I do not think they will all the run out to kill the kids but if one does there is little to stop him and other abuse will even be more likely.

As I said, for various reasons some of them perhaps understandable, the White Council's justice for breakers of the Laws, has become a zero tolerance policy... 

I think that the WC is trying to balance out their place between two worlds, the mortal and the supernatural.

I think it more about finding themselves less relevant in a modern world, the time of the village wise woman or man has past.   Kids who show signs of talent no longer have any place to go or any knowledge of what it all means.  All they know is they have these cool talents and like normal kids everywhere they experiment with them and get into trouble.  Meanwhile the WC has been decimated by wars over the last thousand years and have spent their time navel gazing unlike Harry who has attempted to fit into the modern world openly as a practicing wizard.   As a result, zero tolerance has evolved into not just being the safest course for both worlds but the easiest to implement.  But as we all know zero tolerance policies are seriously flawed because there isn't a one size fits all and justice usually isn't served by it... But those those who advocate it will argue, it keeps everyone safe so it doesn't matter that perhaps a few talented babies who might make great wizards someday are thrown out with the bath water.