How does Irving use two narrators to defend the tale’s credibility and what details reveal a Romantic fascination with the past/nature?

The italicized opening of Washington Irving's "Rip van
Winkle" is more than mere introduction.  It provides a frame, supposedly explaining how
the tale was found, and defending its credibility. But, unlike the "Custom House" which
provides a most serious and intriguing frame to Nathaniel Hawthorne's The
Scarlet Letter, Irving's frame adds mostly humor to his story since
Knickerbocker is clearly an eccentric character of Irving's
imagination.

Aspects of Romanticism are, for
instance, Irving's dwarfing of Rip's domestic problems by the beauty and majesty of
nature.  Lost in the Katskills, Rip van Winkle is calmer and contemplative, much like
the Romantic.  Certainly, Irving's own Romantic ideals are expressed in his satire of
his contemporary post-revolutionary society,  which he suggests is too argumentative,
rationalistic, and dogmatic:

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In place of these a lean bilious-looking fellow
with his pockets full of handbills, was haranguing vehemently about rights of
citizens--elections--members of Congress--liberty--Bunker's hill--heroes of
Seventy-six--and other words which were a perfect Babylonish jargon to the bewildered
Van Winkle.

When Rip arrives,
the bystanders shout "A Tory!...a spy!  a Refugee!"  Rip inquires about his old friends,
only to be told that they have died or gone off.

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Rip's heart died away at hearing of these sad
changes in his home and friends, and finding himself thus alone in the world--every
anwser puzzled him too by treating... of time and of matters which he could not
understand....

As a Romantic,
Irving expresses nostalgia for the stability, calm, and natural beauty of the colonial
village.