the mythology of the Knife is that the wielder cannot be defeated.
True, but I have a hard time believing that this is true in the Dresdenverse. For one thing, it destroys all tension--Harry has the Knife, ergo he will always win. For another thing, it undermines free will somewhat if the wielder will win no matter what anyone does. The Knife is clearly connected to the the White God, and he doesn't do that. What I think the knife might do is allow its wielder to carry out their choices unimpeded, ex. if Harry chooses to look for a way out of the Winter Knight mantle, Mab can't just put a geas on him to stop him, but Harry has to make that choice (and then all the choices that would lead to him finding a way and then choosing to go through with it--although he might be helped by contrived coincidence). If Harry, on the other hand, chose to get into a sword fight with Nicodemus without protective armor, the Knife might tilt things in Harry's favor but I have a hard time believing that it would stop him from getting stabbed.