The redemption at the hands of the Gentiles will come after their period of scattering, and after a "marvelous work" has been done among them which will change their hearts. Clearly this refers to the history of the United States, with their scattering of the Native Americans and the subsequent restoration of the gospel among them.
This "marvelous work," this restoration of the gospel will "be of great worth unto our seed." Perhaps Nephi sees as a given that the marvelous work will be of benefit to the Gentiles, but he sees it in direct relation to the people with whom he is most concerned, his own seed. For them, it will be an act of great worth, so great a worth that the restoration of the gospel, and the preaching of the gospel to the Native Americans will be "likened unto [his seed] being nourished by the Gentiles and being carried in their arms and upon their shoulders." Thus the restoration (and subsequent spread of the gospel) is the very fulfilment of the prophecy of Isaiah.