Oliver Cowdery’s correction here in the printer’s manuscript is not immediate (the level of ink ﬂow for the supralinearly inserted s is weaker). The correction comes at the end of the line in 𝓟 (see line 30 on page 112 of 𝓟). Perhaps Oliver’s correction was the result of proofing 𝓟 against 𝓞 (which is no longer extant here).
This plural phrase “preparations for war” is the dominant one in the Book of Mormon text. Elsewhere in the text, there are seven occurrences of the expression “to make preparations for war” but none of “to make preparation for war”:
In the first of these examples (Alma 24:4), the 1830 compositor accidentally set the singular preparation, but his copy-text, the printer’s manuscript, read in the plural. The 1837 edition restored the correct plural. There is, it would appear, a tendency to replace the plural preparations with the singular preparation.
When we move beyond the specific instances of “to make preparations for war”, we find that there is one more example of the plural “preparations for war”:
We notice that this example uses the quantifier all with the plural preparations, just as in the corrected text for Jarom 1:8 (“and all preparations for war”). On the other hand, there is only one example of the singular “preparation for war” in the text:
Here 𝓟 and all the printed editions except the 1906 LDS edition have the singular preparation (𝓞 is not extant for the word). For discussion of this one case where the earliest text has the singular, see under Alma 49:9.
Summary: Maintain the plural preparations in Jarom 1:8, Oliver Cowdery’s corrected reading in 𝓟; elsewhere the Book of Mormon text consistently uses the plural preparations in the expression “to make preparation(s) for war”.