It appears that in order to make the subject sufferings agree with the third person singular verb form doth, the 1920 LDS edition changed the plural sufferings to the singular suffering. The change was marked in the 1920 committee copy, so we know it was intentional. Of course, another possible emendation would have been to keep the plural sufferings and change doth to do. As explained for the phrase “Nephi’s brethren rebelleth against him” (see under the preface for 1 Nephi), the original Book of Mormon text allowed the third person singular ending -(e)th to occur with plural subjects. Thus here in Moroni 9:19 there is nothing textually wrong with doth occurring with the plural sufferings. The critical text will restore the original plural sufferings and maintain doth.
It is worth noting that the singular noun suffering never occurs in the original text of the Book of Mormon; there are examples of only the plural sufferings (19 of them, including the one here in Moroni 9:19). For further discussion of the consistent use of the plural sufferings throughout the Book of Mormon text, see under Alma 16:19. In contrast, the text has only the singular noun long-suffering (17 of them), never long-sufferings.
Summary: Restore the original plural sufferings in Moroni 9:19 even though the associated verb form doth is in the singular; the original Book of Mormon text allows such language as “sufferings ... doth exceed”; moreover, there are no examples in the original text of the singular noun suffering.