Here we have two cases of the word prophet(s), and both have involved variation. In the first case, Oliver Cowdery initially wrote down in the original manuscript the plural prophets, possibly because the plural is expected, given the preceding plural pronoun they. Somewhat later Oliver crossed out the s with heavier ink flow. Perhaps the correction was made when Oliver read back the text to Joseph. In any event, the singular seems to be correct for the first occurrence of prophet(s). The pronoun they (“they were made manifest”) relates to the brothers’ question found just before in verse 1 (“what mean these things which ye have read”). Nephi had just finished reading a long passage from the prophet Isaiah (Isaiah 48–49 in 1 Nephi 20–21), so in the first case the singular prophet is more appropriate than the plural prophets.
In the second case of prophet(s), Nephi expands the scope of his remarks to say more generally that all things are made known to the prophets by the Spirit. The expansion extends to “all things”, not just the things that Nephi has just finished reading to his brothers. Moreover, Nephi now refers to all prophets, not just one prophet (Isaiah). And since prophets prophesy throughout time, Nephi uses the present tense rather than the past tense to refer to their prophecies (“for by the Spirit are all things made known”). Thus the plural prophets appears to be correct in the second case. The 1830 compositor, probably influenced by the preceding singular prophet, also set the second one in the singular. But the 1837 edition restored the plural prophets, probably by reference to the printer’s manuscript.
Summary: Maintain in 1 Nephi 22:2 the singular prophet followed by the plural prophets.