Here it would appear that Joseph Smith originally intended to change hath to has but ended up writing hath, perhaps because he changed his mind and decided that the has sounded strange in this highly scriptural and archaic sounding expression (“thus hath the Lord commanded me”), especially with its inverted word order. Nonetheless, the 1837 edition ended up replacing the hath with has, perhaps because Joseph Smith’s hath was interpreted as his desire to change hath to has.
Elsewhere the Book of Mormon text has only hath in the inverted declarative expression “hath the Lord”:
Thus Joseph Smith would have been correct to resist changing hath to has in Mosiah 12:1. In fact, the hath was restored in the 1840 edition, with the result that the RLDS textual tradition has continued with the original hath; the LDS text, on the other hand, has maintained the has (the LDS text derives through the 1841 British edition, which used the 1837 edition as copy-text). For further discussion of hath versus has, see under infl al endings in volume 3.
Summary: Restore the original hath in “thus hath the Lord commanded me”, which also agrees with Joseph Smith’s apparent decision to retain hath in this expression.