Here the original text read “the heaven” rather than the more expected “the heavens”. Scribe 2 of 𝓟 initially wrote the word in the plural, although miswritten as Havens, which he immediately corrected to the singular by erasing the plural s. As explained under 1 Nephi 12:6, there is at least one other instance in the earliest text of “the heaven” (namely, in Mosiah 12:36). For further discussion of the competition between “the heaven” and “the heavens”, see under 3 Nephi 26:3.
Here in Mormon 9:17, the original manuscript undoubtedly read in the singular as “the heaven” since both the 1830 reading and the immediately corrected reading in 𝓟 read in the singular. Here the text refers to the word of the Lord regarding the creation of “the heaven and the earth”. Such phraseology in the singular is found in the King James Bible when the creation is described:
The critical text will therefore maintain here in Mormon 9:17 the singular “the heaven”. To be sure, the plural “the heavens” is theoretically possible since it is found in other biblical references to the creation, as in Genesis 2:1: “thus the heavens and the earth were finished”.
Summary: Accept the singular number for “the heaven and the earth” in Mormon 9:17 since such language is found in biblical accounts of the creation (for instance, in Genesis 1:1).