How does Irving use his two narrators (Crayon and Knickerbocker) to defend the tale's credibility, and how would you describe the tone in these introductory passages?
The fictional Sir Geoffrey Crayon, narrator, and Diedrich Knickerbocker, Irving's fictional Dutch-American historian, serve as the framers of the tales and essays of The Sketchbook as Irving engages in the combination of sentimental homages and satires that mark his work. Had Irving taken on his subjects without the buffer of these two "filters," his work might have been seen as entirely subjective and alternately maudlin or biting, depending on the tale. The frame tale structure with Crayon and Knickerbocker is seen as a creative flourish that helped Irving establish the legitimacy of American literature.
The tone Irving seeks to establish in the introductory passages is that of an objective observer who appreciates both America and Europe. Moreover, he is careful to project a humble attitude and present himself not as a philosopher, but as a casual tourist:
"I have wandered through different countries and witnessed many of the shifting scenes of life. I cannot say that I have studied them with the eye of a philosopher, but rather with the sauntering gaze with which humble lovers of the picturesque stroll from the window of one print-shop to another; caught sometimes by the delineations of beauty, sometimes by the distortions of caricature, and sometimes by the loveliness of landscape..."
The narrator introduces the idea of his own fallibility to keep his critics at bay; he (disingenuously) claims that he is not an expert.
By using fictitious narrators, Irving is putting distance between himself and the story. This allows him to pretend that he is not the author but received his stories from these other individuals, who cannot be questioned or cross-examined because they no longer live. That is, Irving can pretend that he is reporting what was passed on to him. (Hawthorne does this, too, in The Scarlet Letter.) Of course, the truthfulness of the story is not important for Irving's purpose -- the story is represented as a tale from the writings of Knickerbocker and the reader is invited to take it as such -- a story from long ago which could be true, although it cannot be verified.
The story is presented as a tale from Knickerbocker, and the tone of the introductory comments is tongue-in-cheek. Crayon acts as a reporter who is merely passing along what he knows. He explains that Knickerbocker has been accepted as an accurate historian, though there has been some skepticism. He notes that Knickerbocker is even revered by some, although he may have stepped on a few toes in writing his history. Of course, Irving stands behind all this winking at us!