Here the compositor for the 1849 LDS edition accidentally set doth as doeth, as if the verb do was the main verb rather than the auxiliary verb. In virtually every case in the Book of Mormon text, the auxiliary form is doth and the main verb form is doeth. For a summary discussion, see under Mosiah 4:18. Also see under infl al endings in volume 3.
Summary: Maintain the auxiliary form doth in Mosiah 29:40, which is consistent with usage elsewhere in the text.