In this exegesis, Nephi is intertwining Isaiah with the vision Nephi had as part of his vision of the Tree of Life. Thus, the context is the distant future, a time when the gathering will be prepared. Isaiah spoke of a scattering and a future gathering, and that is the context in which Nephi interprets these passages at this time. He will use them in a different context when he has Jacob speak on them in the sermon beginning in 2 Nephi 6.
Here, Nephi elaborates on Isaiah according to his understanding of his own prophecy. Thus, Nephi speaks of the salvation to come through gentiles as a reference to the book of scripture that they will have. It will be in a future time that there would be a nation of gentiles who would scatter the Nephites. This personalizing of Isaiah is clearly due to Nephi seeing his vision against Isaiah’s writings. This process, using Isaiah in connection to Nephi’s vision, will be reprised and expanded in 2 Nephi.
The marvelous work that Nephi references will be the revelation of the Book of Mormon to his own people, and he specifically says that “it shall also be of worth unto the gentiles.”