Isaiah 52:12 (King James Bible) and the God of Israel will be your rereward
The 1830 edition follows the King James Bible’s archaic spelling rereward for rearward, meaning ‘rear guard’, not only here in 3 Nephi 20:42 but also in the next chapter:
This spelling suggests that John Gilbert, the 1830 typesetter, checked his copy of the King James Bible in order to come up with such an archaic spelling. Another possibility, at least worth considering, is that the original manuscript itself had this archaic spelling. If so, this could explain why scribe 2 of 𝓟 wrote reward in 3 Nephi 20:42 when he copied the text from 𝓞 into 𝓟 (reward is directly contained within rereward ). On the other hand, Oliver Cowdery, when he proofed 𝓟 against 𝓞, corrected reward to rearward, not rereward, which argues for the spelling rearward in 𝓞. In any event, subsequent Book of Mormon editions have had only the more transparent spelling rearward. This modernized spelling should undoubtedly be continued in the standard Book of Mormon text; the critical text will also maintain this spelling. In fact, elsewhere in the critical text the word will be spelled rearward whenever Isaiah 52:12 is cited.
Summary: Retain in 3 Nephi 20:42 and 3 Nephi 21:29 the more modern spelling rearward in place of the archaic King James spelling rereward.