Here we have a correction in both manuscripts that parallels the correction of is to are earlier in verse 13: Oliver Cowdery wrote the singular word in 𝓞, with the result that when he copied the text from 𝓞 into 𝓟, he initially wrote word but then virtually immediately corrected it to words. Finally, he made the same correction in 𝓞. In both manuscripts, the plural s was inserted inline after the singular word. The level of ink flow for the correction in 𝓟 is without change, but in 𝓞 the s is distinctly heavier, thus arguing for its secondary nature.
In verse 13, the change of is to are appears to have been purely grammatical; that is, the original text seems to have read is. Here in Alma 37:20, there is strong evidence that words is the reading of the original text. The reading all my word seems impossible; it occurs nowhere else in the Book of Mormon (nor in any of the other scriptures for that matter). On the other hand, all my words occurs three other times in the Book of Mormon:
There are two other examples in the history of the text where “all the words” has been accidentally replaced by “all the word”:
In the first example, 𝓞 is extant and correctly reads words; when Oliver Cowdery copied the text into 𝓟, he initially wrote word but then virtually immediately inserted the plural s inline (there is no change in the level of ink flow). Discussion under 2 Nephi 5:4 shows that there are 19 instances in the original text of “all the words” but none of “all the word”. In addition, there are ten occurrences of “all his words” in the text as well as one each of “all thy words” (Alma 18:23) and “all their words” (Mosiah 12:19). But there are none of “all his word”, “all thy word”, or “all their word”.
Finally, there is independent evidence that Oliver Cowdery tended to write words as word in the manuscripts, if only momentarily. In one very clear example, Oliver initially wrote these word in 𝓞 but then corrected the manuscript to these words:
For other examples of this same error, see under 2 Nephi 29:2–3 and Mosiah 22:4. The critical text will therefore accept Oliver Cowdery’s correction of word to words in Alma 37:20 as the original reading even though it apparently resulted from his own conscious editing.
Summary: Accept in Alma 37:20 Oliver Cowdery’s correction of word to words in the manuscripts since the singular is essentially impossible in the phrase “all my word(s)”.