An easily overlooked part of the Book of Mormon regarding this issue is when God gave this land to the Lamanites, and prophets prophesied that the gentiles would have temporary, conditional ownership of part of it, while the Lamanites would have indefinite, unconditional claim on it. I don’t know how much of a factor that plays in it, but the church has always opposed immigration laws that separate families (they’ve made statements about it before). And regarding any BoM statements of borders, the only time borders mattered were when wicked people wanted to take over the land to destroy their freedom. We’re that the case here, obviously we’d see a much different scenario.