Here in both manuscripts, Oliver Cowdery initially wrote the singular man (“and bringeth about means unto man”). In both instances, Oliver overwrote the original a letter with an e, and there is no apparent difference in the level of ink flow for the overwriting. One possibility here is that in both cases Oliver mistakenly wrote men as man and then virtually immediately corrected his error. Here is one example where it is clear that Oliver sometimes mistakenly wrote man instead of men and corrected it immediately:
Initially in 𝓞, Oliver wrote “the diligence of man”; then he overwrote the a with e (as here in Alma 34:15). But the resulting word, men, seemed unclear, so Oliver crossed it all out and wrote inline the correct plural men. Thus in Alma 28:14 we have an immediate correction.
On the other hand, it is also possible in Alma 34:15 that Oliver Cowdery’s correction in 𝓞 and 𝓟 of man to men was in both instances due to conscious editing, although virtually immediate. The word man is followed only two words later by the plural pronoun they (“and bringeth about means unto man that they may have faith unto repentance”). The jarring violation in number could have led Oliver to quickly emend man to men. It’s even possible that he made both changes when he was copying from 𝓞 into 𝓟. And we can also find independent evidence in the manuscripts that Oliver would sometimes consciously emend man when it was surrounded by instances of the plural men or plural pronouns referring to men:
In this instance, 𝓞 is sufficiently extant to determine that it read man, which is what Oliver initially wrote in 𝓟. Yet here he seems to have soon, if not virtually immediately, corrected man to men. There is, once more, no apparent change in the level of ink flow for the correction, although in this case Oliver did not simply overwrite the a with an e; instead, he crossed out the entire word man and supralinearly inserted the plural men. These conflicting sources for the manuscript correction of man to men (Alma 28:14 versus Alma 29:4) make it difficult to decide in Alma 34:15 whether the original text read man or men.
Considering other verses that contain the word means, we find that either man or men can occur, including one instance (marked below with an asterisk) where the verse has both:
Of course, one could argue that the unique use of men, in Alma 42:12, could be an error for man. But that passage is extant in 𝓞 and shows this variability between man and men.
We should also note that there are other instances of man, inevitably with the meaning ‘mankind’, that take a plural pronoun. The following instances are interesting in that the plural pronoun follows closely upon man, yet Oliver Cowdery never emended any of these instances of man to men:
These examples show that Oliver Cowdery didn’t make the emendation of man to men particularly often, but we know he did it at least once (in 𝓟 for Alma 29:4) and possibly here in both 𝓞 and 𝓟 for Alma 34:15.
So it is very difficult to decide the case of man versus men in Alma 34:15. One suggestive difference is that for the immediate correction of man to men in Alma 28:14, Oliver Cowdery initially emended man by overwriting the a with e, while in the case of the one clear case of emendation (in Alma 29:4), Oliver corrected man to men by crossing out the entire man and then supralinearly inserting men. Since here in Alma 34:15, the correction is by overwriting the a with e (and in both 𝓞 and 𝓟), the critical text will accept the plural men as a correction to the actual text and not as a deliberate emendation.
Summary: Accept in Alma 34:15 the virtually immediate correction of man to men in both 𝓞 and 𝓟, although it is almost equally probable that this correction was due to editing on Oliver Cowdery’s part.