As explained under Mosiah 12:20, original instances of meaneth and doth mean with plural subjects will be maintained or restored, as the case may be, in the critical text. The original text had many instances where plural subjects took verbs ending in the historical third person singular ending -(e)th. In the Book of Mormon, this ending was apparently used as an indicator of the biblical style. For a complete discussion, see under infl al endings in volume 3. Here in Helaman 5:38, the 1840 edition made the grammatical correction to the plural do; this reading entered the LDS text in the second printing of the 1852 LDS edition, apparently by reference to the 1840 edition. On the other hand, the 1908 RLDS edition restored the original doth to the RLDS text (apparently by reference to 𝓟).
Summary: Restore the original doth in Helaman 5:38 even though its subject is the plural noun phrase “all these things”.