The first sentence of chapter 29 does not make any sense in the current configuration of chapters (and there is no chapter break at this point in the 1830 edition). By separating verse 29:1 from verse 28:32, we have lost the reference and the reason for “but behold, there shall be many.” Without that previous verse providing context, it is unclear of whom there shall be many. Coming on the heels of 28:32, however, the context is the Gentiles:
2 Nephi 28:32 Wo be unto the Gentiles, saith the Lord God of Hosts! For notwithstanding I shall lengthen out mine arm unto them from day to day, they will deny me; nevertheless, I will be merciful unto them, saith the Lord God, if they will repent and come unto me; for mine arm is lengthened out all the day long, saith the Lord God of Hosts.
2 Nephi 29:1 But behold, there shall be many….
Verse 28:32 is a condemnation of the Gentiles, save they should repent. The end of verse 32 provides the “unless” clause, and then verse 29:1 picks up the theme of the repentant Gentiles, and indicates that “there shall be many.” The rest of the verse provides the time context for these repentant Gentiles - the last days when God will set his hand again to recover his people.
Textual: One of the important shifts in the narration is the change in voice. Where Nephi has been recounting his prophecies, we have in this section a first person account from the Lord. This shift began with the citation of Isaiah in 2 Nephi 28:30. In Isaiah’s text, this theme is in Isaiah’s voice. When it appears in Nephi’s text, the speaker is the Lord, and there is a shift in the text from Nephi’s recounting of his vision, to a first person discourse.
Of course this revelation from the Lord comes through Nephi, but so also were the revelations that Nephi recounted in the past tense and with his own words. This revelation is either a citation, or a revelation that is being given as Nephi is writing. There is no way of knowing when Nephi received the revelation that is recounted here. It is certainly thematically related to the rest of Nephi’s discourse during this pesher on the Isaiah chapters. At the very least, the shift in the locutor allows Nephi to highlight themes that have already been presented, but do so in a slightly different context.
Because the verses that follow show the same modes of referencing Isaiah, including the oblique literary references, the revelation fits into the style of Nephi. Of course, the Lord would likely communicate with a prophet through his own understanding and style, so this is still no determinant of the timing of the revelation.