Here in verse 24, the earliest text reads oddly. It appears that the if- clause, “and if it so be that they perish”, is unrelated to the following main clause. It is as if the subject matter has been changed, with the main clause referring to Nephites who are surviving, not perishing, by dissenting over to the Lamanite side. Note that earlier in verse 23, the main clause following the if- clause appropriately completes that sentence: “and if they perish / it will be like unto the Jaredites”.
The 1906 LDS large-print edition attempted to deal with the seemingly unrelated if- clause in verse 24 by changing the subordinate conjunction that to the conditional conjunction then: “and if it so be / then they perish”; in other words, we now have an if- clause (“and if it so be”) followed by a different main clause (“then they perish”). It may be helpful here to compare the readings in the 1879 and 1906 LDS editions, along with their accidentals, since the 1879 edition served as the copytext for the 1906 edition:
Note that the editors for the 1906 edition added dashes to the preceding parenthetical information that ends verse 23 (that is, around “because of the willfulness of their hearts / seeking for blood and revenge”) but left unchanged the punctuation for the revised expression in verse 24. In other words, they did not show that the statements about the dissenters were independent of the newly created if-then construction. Perhaps a stop, such as a period, could have been placed there:
The 1907 LDS vest-pocket edition followed the 1906 emendation of that to then. The copytext for that edition was still the 1879 LDS edition, but in verse 23 the 1907 edition attempted to use both the dashes of the 1906 edition as well as the commas of its copytext, with some alteration. But in verse 24, like the 1906 edition, the 1907 edition continued with the original punctuation:
Ultimately, here in Moroni 9:23–24 the LDS text has maintained the earlier text and accidentals of the 1879 edition (the text for the 1981 edition derives from the 1905 edition rather than from the 1906 or 1907 editions).
David Calabro (personal communication) points out that the earliest text here in Moroni 9:24 can be dealt with if we realize that here Mormon is stating that even if all those identified as Nephites perish, there will nonetheless be dissenters from the Nephites, ones who will deny Christ, who will survive but as Lamanites (that is, as nonbelievers). Mormon refers to this situation, after the final great battle at Cumorah, when he mentions a few dissenters who have survived:
And Moroni specifically refers to the fact that he could survive if he would deny the Christ (thus becoming a dissenter):
And earlier in the text, Mormon more generally refers to Nephites who will dissent over to the Lamanites, and in this way a remnant of the Nephites will survive:
So under this interpretation, Moroni 9:24 makes sense. Even though the Nephites will perish as a nation, some of their people will survive as Lamanites. In other words, there is a contrastive aspect to Mormon’s statement here in Moroni 9:24. Since this interpretation will work, the critical text will accept the earliest (and current) reading in Moroni 9:24, where the if- clause is actually completed by the main clause that refers to the dissenters.
Summary: Despite its difficulty, the if- clause in Moroni 9:24 is completed by the main clause referring to dissenters; here Mormon is stating that not all the Nephites will perish, only the nation of people identified as such; dissenters who join the Lamanites will survive.