The original manuscript has the singular labor (spelled as labour) in the phrase “the fruit of their labor”. In the printer’s manuscript, Oliver Cowdery accidentally replaced the singular labor with the plural labors (spelled as labours), thus giving “the fruit of their labors”. The plural labors (sometimes spelled as labours) has been retained in all the printed editions.
The Book of Mormon text has three other examples of the phrase “the fruit(s) of one’s labor(s)”, and in each case the original text has both fruit and labor in the plural:
In Alma 36:25, Oliver Cowdery accidentally replaced the plural fruits with the singular fruit as he copied the text from 𝓞 into 𝓟. These four examples of “the fruit(s) of one’s labor(s)” show that in the original text fruit and labor always agree in number: either both are singular (as here in Alma 29:17) or both are plural (in Alma 26:31, Alma 36:25, and Alma 40:26).
The completely singular case in Alma 29:17, although unique in the Book of Mormon, is probably intended. In support of this reading with two singulars, we note that the only parallel example in the King James Bible has both fruit and labor in the singular: “this is the fruit of my labor” (Philippians 1:22). The critical text will restore the singular labor in Alma 29:17, the reading of the original manuscript.
Summary: Restore in Alma 29:17 the singular labor, the reading in 𝓞 (thus “the fruit of their labor”); similar usage is found in the King James Bible.