The printer’s manuscript has the plural phrase “the promises of the Lord were”. Oliver Cowdery initially wrote promises in 𝓟 as promisee, then overwrote the repeated e with an s. In the original manuscript, the ro of promises and the plural verb were are extant. Most probably 𝓞 had the plural promises since the verb is the plural were. The noun immediately preceding were is the singular Lord, which could have theoretically led to “the promises of the Lord was”, as originally in 2 Nephi 10:9: “the promises of the Lord is great unto the Gentiles” (see the discussion under that passage). Here in Alma 48:25, the earliest textual sources support the plural reading “the promises of the Lord were”.
The 1830 edition changed promises to the singular promise but retained the plural verb were (“the promise of the Lord were”), which suggests that the loss of the plural ending in the 1830 edition was simply a typo rather than the result of editing. The 1837 edition removed the disagreement in number by changing the were to was rather than restoring the plural promises. But Joseph Smith, in his editing of 𝓟 for the 1837 edition, inserted inline an extra plural s after promises, perhaps in an attempt to guarantee the plural promises.
Ultimately, the question here in Alma 48:25 is whether we have a single promise or more than one promise. Elsewhere the text has two passages that specifically use the noun promise to refer to the Lord’s statement that if the people keep his commandments they will prosper in the land; one of these instances uses the singular promise, the other the plural:
Thus the occurrence in Alma 48:25 of the plural promises is possible, and the critical text will restore the original “the promises of the Lord were” in this passage, the original reading in 𝓟 and the probable reading in 𝓞.
Summary: Restore in Alma 48:25 the plural usage in “the promises of the Lord were”, the earliest reading in the manuscripts; in addition, Mosiah 1:7 shows that the plural promises is an appropriate reading for Alma 48:25.