In this passage, the first instance of the prepositional a was edited out of the text. Initially in 𝓞, Oliver Cowdery missed the a, but then virtually immediately he supralinearly inserted it (there is no change in the level of ink flow for the supralinear a). Then some time later, this a was crossed out, apparently by Oliver Cowdery; the crossout is with somewhat heavier ink flow. Perhaps Oliver decided to emend the text here since the present-participial mourning in the preceding phrase lacks the prepositional a (“yea the cry of widows mourning for their husbands”).
Later on in this verse, we have a second instance of a mourning, and in this case both 𝓞 and 𝓟 have the prepositional a: namely, “a mourning for their kindred which had been slain”. Initially in 𝓟, Oliver Cowdery once more omitted the a and, as before, supralinearly inserted it. In this case, he had earlier written the a in 𝓞 and without any correction; nor did he remove this second instance of the prepositional a in either manuscript. Don Brugger (personal communication) suggests that this second instance of a mourning could be interpreted as a gerund in apposition to the preceding “the cry of mourning”. This possible difference in grammatical interpretation could have led Oliver to leave this case of a mourning in the text. In any event, the 1830 compositor interpreted the a here as a case of the prepositional a and decided to remove it when he set the type. The most reasonable interpretation is that “a mourning for their kindred which had been slain” is a present participial clause that modifies the preceding noun phrase “every one of them”, just like the two earlier instances of present participial “(a) mourning” modify their preceding nouns, widows and fathers.
David Calabro points out (personal communication) that in the original text the very first occurrence of the participial mourning (“yea the cry of widows mourning for their husbands”) may have also read with the prepositional a (as “yea the cry of widows a mourning for their husbands”), even though 𝓞 is extant in this particular case and reads without the a. Further support for this possibility comes from two very similar occurrences of present-participial mourning earlier in the text where the printer’s manuscript twice reads “a mourning for X”:
Not surprisingly, the 1830 typesetter removed these two instances of the prepositional a from Mosiah 21:9. On the other hand, there is one other example of present-participial mourning; 𝓞 is extant in this case and lacks the a:
Typically, present participial verb forms lacked the prepositional a in the original Book of Mormon text. For instance, there are six instances of going forth in the text, of which only two originally had the prepositional a (each marked below with an asterisk):
Thus the text allows for variation in the use of the prepositional a. In each instance, we rely on the earliest extant text to determine whether the a is there or not. The critical text will therefore restore both confirmed instances of the prepositional a in Alma 28:5. The deletion of the supralinearly inserted a in the first instance appears to be due to editing. For additional discussion regarding the prepositional a in the Book of Mormon text, see under 1 Nephi 8:28. For a complete analysis, see under prepositional a in volume 3.
Summary: Restore both cases of the original prepositional a in Alma 28:5: “a mourning for their sons … a mourning for their kindred which had been slain”; on the other hand, the first instance of mourning in this passage will be retained without the prepositional a: “yea the cry of widows mourning for their husbands”, as will the one in Alma 28:11: “yea and many thousands are mourning for the loss of their kindred”.