At the beginning of this passage, the printer’s manuscript has “and then”, but the 1830 edition has only the then. As already noted, the overwhelming tendency in the history of the Book of Mormon text has been to accidentally drop words rather than to add them (especially when the scribes or the typesetters were only attempting to copy the text rather than edit it). For this reason, the original manuscript here in 3 Nephi 20:46 probably read “and then”, which means that the 1830 compositor accidentally missed the and that would have been written as an ampersand in 𝓞 since Oliver Cowdery was the scribe for this part of 𝓞 (two nearby fragments of 𝓞 in 3 Nephi 19–21 are in Oliver’s hand).
Despite this argument from transitional probabilities, it is possible that scribe 2 of 𝓟 wrote “and then” because of the subsequent instance of “and then” later on in this passage: “and then shall Jerusalem be inhabited again with my people”. His eye could have glanced down to the following line in 𝓞, thus prompting him to accidentally insert an extra and. Yet elsewhere we have no examples where scribe 2 of 𝓟 ever accidentally added an and, even momentarily; there are a number of cases where he omitted an and—and usually without him catching his error (six out of nine times). As we might expect, the 1830 compositor sometimes omitted the and; in each of the following examples, the omission appears to be accidental:
The critical text will therefore accept the reading in 𝓟 as the original reading here in 3 Nephi 20:46 (“and then shall this covenant ... be fulfilled”); the chances are much greater that an and was lost here than added.
Summary: Restore the and in 3 Nephi 20:46 (“and then shall this covenant ... be fulfilled”), the reading of the printer’s manuscript; the 1830 compositor seems to have accidentally skipped an ampersand as he set the type from 𝓞.