The original manuscript is not extant here for hand(s). The printer’s manuscript has the plural hands, which was retained in the text until the 1920 LDS edition, when it was replaced by the singular hand. This emendation is in agreement with every other occurrence of “by the hand(s) of the Lord” in the Book of Mormon; excluding this case in Alma 46:7, there are 20 occurrences of “by the hand of the Lord” but none of “by the hands of the Lord”. Moreover, whenever “by the hands of X” does occur in the text (where X is not “the Lord”), X is always a plural and refers to people:
Thus “by the hands of the Lord” in Alma 46:7 is doubly exceptional. Finally, we should note that in “by the hand of X” (where X refers to people), X can be either singular or plural providing X does not refer to the Lord or to God; there are ten occurrences of singular X and eight of plural X.
Once more, there are two possibilities for the original manuscript: (1) the text read hand, but Oliver Cowdery accidentally copied it as hands into the printer’s manuscript; or (2) the text in 𝓞 read hands, but this was nonetheless a mistake for hand. Either possibility is consistent with Oliver’s tendency to accidentally add plural s’s. Since here the 1830 signature was proofed against 𝓞 (see under Alma 42:31), one could argue that 𝓞 read hands since the 1830 reading is hands. But one could also argue that 𝓞 read hand and that the minor difference between hand and hands was simply missed during proofing.
For a list of examples where hand and hands have been mixed up in the history of the text, see under Mosiah 16:1. Of particular interest here in Alma 46:7 is the following nearby error in 𝓞 where Oliver Cowdery initially wrote hands rather than hand:
This phrase, “by the hand of God”, is essentially the same as the phrase here in Alma 46:7 (“by the hand of the Lord”). Thus we see that Oliver tended to accidentally replace hand with hands in phrases referring to “the hand of the Lord” or to “the hand of God”. It doesn’t matter much whether Oliver’s error in Alma 46:7 occurred when he took down Joseph Smith’s dictation (in 𝓞) or when he copied the text from 𝓞 into 𝓟. Textual consistency strongly argues that Alma 46:7 should read “by the hand of the Lord”, in accord with the emendation in the 1920 LDS edition.
Summary: Accept the emendation in the 1920 LDS edition that replaced hands with hand in Alma 46:7; usage elsewhere in the text strongly supports the phraseology “by the hand of the Lord”; moreover, Oliver Cowdery made the same basic error in 𝓞 for Alma 46:24, but there he immediately caught his error.