On the matter of Skin Game, no one was lying at any time. There were mutual obligations involved and they were acting as courtesy demanded. Nic knew that Mab was out to end him. Mab knew Nic would kill Harry as soon as he had what he went to the vault to get. Hades needed to know that Harry could win in a contest against Nicodemus in this context. Mab actually states it explicitly.
But Harry was lied to by Mab, the lie of omission because she never leveled with him from the beginning. She kept Molly away from him because she'd have told him the truth of what was really going on. That is what the final confrontation between Harry, Molly, Marcone and Mab was all about at the end of Skin Game.
"You could have told me from the beginning," I said to Mab.
He then goes on, on the next page.
Then Mab finally spoke, her voice sepulchral. "Do you have a point, my Knight?"
"I wanted to let you know that I knew,"I said.
He didn't say it outright, but the implication is clear, he was lied to from the beginning and as a result an innocent died, Murphy was severely crippled, and a Holy Sword was broken. It doesn't matter that Nic knew that Mab wanted to end him, the mission was a set up supposedly as far as
Harry knew to repay a favor, which put him under constraints from the beginning. Mab still lied to him. That is why Molly was there, she was backing him up, because Mab made her part of the lie by keeping her away from Harry.
That is why Mab appears to be so pissed in this scene, she had lied to her Knight and he called her on it.