Here in the printer’s manuscript, Oliver Cowdery initially wrote “with our mights”. He immediately corrected the mights to might by erasing the plural s. At the same time, it would appear, he supralinearly inserted the word all (there is no change in the level of ink flow for the all ). The original manuscript is extant for the last part of the word might, and the reading is might, not mights. In addition, spacing between extant fragments strongly supports the occurrence of the all in 𝓞.
As explained under Jacob 1:19, the plural usage “with our mights” is actually characteristic of the original text of the Book of Mormon; although the plural “with one’s mights” is not required, it is much more frequent in the original text than “with one’s might” (11 occurrences to 2 in plural contexts). On the other hand, we always get the singular might when all is a premodifier, as in Jacob 7:25: “the people of Nephi did fortify against them with their arms and with all their might”. (In all there are five examples of “with all one’s might”, including the one here in Alma 58:13).
Summary: In accord with the corrected reading in 𝓟 as well as with what is extant in 𝓞, maintain in Alma 58:13 the earliest extant reading “with all our might”; for this phrase, the singular might is consistent with usage elsewhere in the text.