Here the editors for the 1920 LDS edition changed “more exceeding numerous” to “exceedingly more numerous”. The change from exceeding to exceedingly was common in the 1920 edition. For discussion of that change, see under 1 Nephi 2:16 (or more generally under exceeding in volume 3). The more crucial change here in Helaman 4:25 is the shift in word order, where more was moved from before exceeding to after it. Basically, the earliest reading states that both the Nephites and the Lamanites were “exceeding numerous”, but the Lamanites more so.
Elsewhere the original text has only the other word order, “exceeding more ”; it turns out that the adjective in this phrase is always the word numerous:
The first of these, in Jarom 1:6, is particularly relevant in interpreting Helaman 4:25 since the comparison there is between the population of the Nephites and the Lamanites—and the Lamanites are “exceeding more numerous” than the Nephites. Thus one could consider the word order “more exceeding numerous” in Helaman 4:25 as an error that occurred during the early transmission of the text, although there is not much evidence in the history of the text for changes in word order of this nature. There is one example, in 1 Nephi 17:41, where two adjectival modifiers were switched in order; in that instance, the original order “flying fiery serpents” was changed to “fiery flying serpents” when Oliver Cowdery copied the text from 𝓞 into 𝓟. But it should be noted that these two words, flying and fiery, are visually similar, unlike exceeding and more in Helaman 4:25.
The word order “more exceeding(ly)” actually does occur in the Book of Mormon text, but only once, and that is in an adverbial phrase:
The usage here implies that Zeezrom had already been trembling exceedingly, but now even more so. And at the beginning of Alma 12, we can find support for Zeezrom’s trembling:
Presumably, Zeezrom’s trembling in verse 1 was already exceeding, so by verse 7 he “began to tremble more exceedingly”. Although not used in Alma 12:1, there are passages that specifically refer to someone as “trembling exceedingly”:
It should also be pointed out that there are examples of “more exceeding(ly)” in the King James Bible:
In the last one, as pointed out by Don Brugger (personal communication), the phrase “more exceedingly” modifies an adjective, zealous.
So ultimately the question here in Helaman 4:25 is whether the population of the Nephites can be considered “exceeding numerous”. And we can find evidence elsewhere in the text that the Nephites are indeed numerous (one of which refers to them as being “very numerous”):
The Nephites are numerous, yet in the book of Mosiah the Lamanites are referred to as being more than twice as numerous (even with the people of Zarahemla having now joined with the people of Nephi):
Thus one can refer to the Lamanites as being “more exceeding numerous” than the Nephites. The critical text will therefore maintain the earliest extant reading in Helaman 4:25, the reading in 𝓟.
Summary: Restore in Helaman 4:25 the original word order, “more exceeding numerous”, as well as the form exceeding in place of the innovative modern adverbial form exceedingly; usage elsewhere in the text allows for exceeding to be compared; moreover, the Nephites are considered “very numerous”, so the more numerous Lamanites can be considered “more exceeding numerous” than the Nephites.