The earliest attested text here reads “and secure”—that is, with a present-tense form of the verb, which doesn’t make much sense considering the preceding past-tense verb forms kept and was. The 1849 LDS edition emended the text to read “and secured”, which the LDS text has consistently followed ever since. On the other hand, the RLDS text has retained the earliest but difficult reading, “and secure”.
The 1849 emendation suggests that the original text actually might have read “and secured”— that is, the verb secure ended in the past tense ending -d, but early in the transmission of the text the d was lost. One possibility is that when Joseph Smith dictated this passage, the scribe for 𝓞 had difficulty hearing the final voiced alveolar stop /d/ because the next word began with the voiced interdental fricative /d/. In other words, secured their /sßkjurd der/ was misheard as secure their /sßkjur der/. There is support for this kind of mishearing in Alma 56:37, where internal evidence suggests that the original text read “and as we supposed that it was their intent to slay us before Antipus should overtake them”; here the d at the end of supposed was apparently not heard because of the following that. For further discussion, see under Alma 56:37.
One semantic difficulty with the emendation to secured here in Mosiah 21:18 is that the resulting text seems to treat separately the keeping together of the people and the securing of grains and flocks, with the added implication that the people’s attempts to secure their grain and flocks were successful. Another possible reinterpretation for Mosiah 21:18 is that the people of Limhi kept together as much as possible in order to secure their grain and flocks. Such an interpretation suggests that the original text actually read “to secure”, not “and secured” (or “and secure”), and that the copying error was to accidentally replace the infinitive marker to with the conjunction and, not to drop the d at the end of a supposedly original past-tense form secured.
There are other examples in the text where a conditional clause of the form “it possible” is followed by an infinitive clause:
The example from Alma 56:29 closely parallels the suggested emendation here in Mosiah 21:18. Both take the form “finite clause + conditional clause + resultive infinitive clause”. We also have the following parallel examples in the King James Bible:
There are also some examples of mix-ups between to and & in the early transmission of the text; in the following list, I include examples from the 1830 edition, which was set from manuscript (in which and was typically written as an ampersand):
These examples support the possibility of mixing up to and &, although we do not have an explicit example in the manuscripts of to being miswritten as &.
Summary: Emend Mosiah 21:18 by replacing “and secure” (the reading of the earliest textual source, the printer’s manuscript) with “to secure”; this emendation is supported by usage elsewhere in the text, while the 1849 LDS emendation to “and secured”, although theoretically possible, does not fit the expected semantics for this passage.