The earliest text has a plural head noun (promises) and a singular verb form ( is). The 1830 compositor replaced the is with are. In his editing for the 1837 edition, Joseph Smith marked the change in 𝓟. The same kind of editing is found later on in this chapter, although in this instance the 1830 compositor set the is of the printer’s manuscript:
These specific examples of subject-verb disagreement appear only here in 2 Nephi 10. Elsewhere we always get the correct plural form of the verb be when the subject is “the promises of the Lord”:
The last example shows that the original plural promises in 𝓟 was accidentally misread by the 1830 typesetter as the singular promise, which ultimately led to changing the plural were to was in the 1837 edition. Oliver Cowdery’s copying of the text into 𝓟 involves a correction: he initially wrote promisee (a slip of the pen) but then overwrote the second e with an s. Later, Joseph Smith inserted an s at the end of promises just to make sure that the word would be read in the plural. In any event, the original text in Alma 48:25 definitely read in the plural for both the subject and the be verb (“the promises of the Lord were”), but the 1837 edition ended up with the singular for both the subject and the be verb (“the promise of the Lord was”). For further discussion, see Alma 48:25.
This regular use of the plural form of the be verb elsewhere in the text for “the promises of the Lord” suggests that we should at least consider the possibility that the original text in 2 Nephi 10:9 and 2 Nephi 10:21 had a singular promise but that in the early transmission of the text (𝓞 is not extant here) the singular was replaced by the plural promises. Besides these two examples in 2 Nephi 10 and the four from the book of Alma (listed above), the text has two occurrences of the singular “promise of the Lord” (2 Nephi 3:5 and Mosiah 7:32) and four more occurrences of the plural “promises of the Lord” (Alma 28:11, Alma 28:12, Helaman 15:12, and 4 Nephi 1:49) as well as two occurrences of “the promises which the Lord (had) made” (Mosiah 1:7 and 4 Nephi 1:11). Only the example in Alma 48:25 shows any variation between singular and plural promise(s). The odds are that the two instances of promises in 2 Nephi 10 did not involve any variation in number for the noun itself.
We should note that for only the two examples in 2 Nephi 10 is the verb be a linking verb that takes an adjective as its subject complement (namely, great). This systematic difference may provide some explanation for the difference in subject-verb agreement. (For discussion, see subject-verb agreement in volume 3.) In any event, the critical text will restore these two cases in 2 Nephi 10 where is is the verb for the plural “the promises of the Lord”.
Summary: Restore the use of the singular is with the plural “the promises of the Lord” in 2 Nephi 10:9 and 2 Nephi 10:21; these two instances of “the promises of the Lord” are probably not errors for the singular “the promise of the Lord”.