Ethniu never had to give up power to remain in this reality. She has never left it and simply remained low-key until BG. Remember she is called theLast Titan by Vadderung. So she's just the last of the beings of her power level walking around free. There are greater powers locked in Demonreach according to Harry's thoughts while confronting the HWWB on the boat.

Probably because if they did a Warden would show up to cut their head off for breaking the First Law.

But you would think it would be something all the Wardens learn as part of their combat training. It wouldn't be your main offense, but it a nice surprise trick. And Harry took out a Black Court Elder with it, so we know it's effective.

I don't think Shagnasty has anything that can be seduced.  He's sadistic.  His idea of foreplay would be to rip someone apart and then play with their organs.

Well we know that the naglosii sometimes have those urges, hence Goodman Grey. I doubt seduction is involved though.

I think Chandler's a bad guy working with Drakul. Chandler being sent away through a portal or whatever while others were actually taken prisoner is somewhat telling to me.

Not really. Chandler was done by Drakul, the others by his flunkies. Different methods used by those of different capabilities.

Since the quote comes from Small Favor it's hard to see it's relevance to Proven Guilty. It did occur to me that Jim was trolling his readers and introducing the idea that there was a door into Winter in the Lab and preparing them for a trip down the primrose path.

And that I think answers the question.

Harry "broke" Little Chicago when he was making it. Leah( or Mab if Leah was incapacitated) saw the flaw and fixed it in her godmother mode without telling Harry.

That too, but the fact remains, Mab lied.

Mab didn't lie, she just didn't reveal her whole agenda same as Nic.

Nic bargained for the assistance of the Winter Knight in retrieving an object from Hades vault. He made no promises and asked for none about his plans afterwards because he wanted to be free to kill Harry, whoever Harry brought to watch his back and Anna Valmont in a place where Mab would not be able to intervene. Harry did his part in aiding Nic's plan. Without him, there is no way Anna signs on to Nic's crew and they don't get past the Gate of Ice later. Harry personally gives Nic the item he states was his objective for the job.

Then we get Nic's betrayal and the battles afterwards. Harry, Michael and Anna only survive because Harry made the smart plan to preemptively hire Grey and have him be their ace in the hole. They overcame Nic's hidden agenda, bringing the favor to an end and starting a free-for-all to end the caper. The fight at Michael's was just Nic being a vengeful bad guy.

But Harry(and hence Mab) did keep faith with Nic on his stated bargain. It's just that Nic being treacherous got them so much more.

Mab and Marcone(also Namshiel as we now know) got revenge on Nic for SF.

Mab and Hades got the weapons out of the vault and into capable hands for the future.

Uriel got the preliminary redemption of Nic's squires and a new Knight. He even got Michael more protection by alerting Molly to the holes in his security.

Harry got Bonea out of his head(his bargain with Mab to start the plot) which also let him leave  Demonreach and interact with the world again,  a fortune in diamonds and the mixed blessing of possessing some Christianity's most powerful holy relics.

By planning for Nic's treachery, Mab was able to accomplish more than balancing her books.  And she did it without breaking the letter of their agreed upon bargain.

He also works the angles, what is in it for him?  In Changes he saw nothing in it for him, even to save a child, so he turned Harry down.

He didn't turn Harry down. He gave him the little information that he had and had Gard introduce him to Vadderung.