The extant original manuscript here reads “the commencement of the seventeenth year”. But when Oliver Cowdery copied from 𝓞 into 𝓟, he accidentally omitted “the commencement of ”, thus giving “in the seventeenth year”. The 1981 LDS edition, in accord with the reading in 𝓞, restored the missing phrase “the commencement of ”. The tendency to omit “the commencement of ” is relatively frequent in the text. Out of 17 original occurrences of “it came to pass (that) in the commencement of the Xth year”, six involve the loss (sometimes only momentarily) of “the commencement of ”. There is another example where Oliver omitted this phrase when he copied from 𝓞 into 𝓟 (and as with Alma 30:5, this phrase was restored in the 1981 edition):
In four cases, the phrase was initially omitted in 𝓞 (three times) or in 𝓟 (once, marked here with an asterisk):
In Alma 30:5 the 1981 LDS edition also added the subordinate conjunction that before the in. Of the 16 other occurrences of “it came to pass (that) in the commencement of the Xth year”, three have the that in the earliest text:
For the first two cases, the that is extant in 𝓞. In the third case, 𝓞 is not extant; but for that part of the text, 𝓟 and the 1830 edition are both firsthand copies of 𝓞, and each one has the that. So quite clearly, that can occur in this construction. Of the 13 other occurrences without the that (that is, where the earliest text reads “it came to pass in the commencement of the Xth year”), 𝓞 is extant in five instances. Thus there is clear evidence that this phrase can lack the subordinate conjunction that before the preposition in, so in each case we follow the earliest textual sources in determining whether the that is there or not.
David Calabro wonders (personal communication) if there shouldn’t always be a subordinate that either before or after the commencement phrase. In the 13 clear cases where there is no that before the commencement phrase, there are four instances where that follows (as in Alma 45:20: “and now it came to pass in the commencement of the nineteenth year of the reign of the judges over the people of Nephi that Helaman went forth among the people”). But in the remaining nine cases, there is no following that (as in Alma 62:12: “and it came to pass in the commencement of the thirty and first year of the reign of the judges over the people of Nephi / Moroni immediately caused that provisions should be sent”). It should also be pointed out that theoretically that could both precede and follow the commencement phrase, as can be found in the original text for other adverbial phrases (for instance, Jacob 7:17 originally read “and it came to pass that on the morrow that the multitude were gathered together”). However, in the three established cases where that precedes the commencement phrase (listed above), there is no repeated that (thus Alma 50:25 reads “and it came to pass that in the commencement of the twenty and fourth year of the reign of the judges / there would also have been peace”).
As for the occurrence here in Alma 30:5, spacing between extant fragments of 𝓞 suggests that the subordinate conjunction that was there, at least initially. This possibility is represented as follows in the transcription of 𝓞 in volume 1 of the critical text:
year the sixteenth ^ of the Reign of the Judges over the Peo(p )
LE OF NEPHI & IT CAME TO PASS THAT IN
the commencement of the seventeenth year of the (Reig ) N OF THE JUDGES THERE WAS CONTINU
The possibility that the that in 𝓞 could have been crossed out is mentioned in a footnote to the transcription. Of course, it must be remembered that the that here is conjectured; there could have been some other correction for this missing part of 𝓞. Or perhaps the lacuna is not as long as postulated in the transcription; in other words, there may have been no that at all in 𝓞. Given this inconclusiveness regarding the that in Alma 30:5, the critical text will simply follow the earliest extant reading—namely, the reading in 𝓟, which lacks the that: “and it came to pass in the commencement of the seventeenth year”.
Summary: Maintain in Alma 30:5 the phrase “the commencement of ” that was originally omitted when Oliver Cowdery copied the text from 𝓞 into 𝓟; remove the that which the 1981 LDS edition added when restoring the phrase “the commencement of ” to the text; in this instance, the earliest extant source for the words “and it came to pass (that) in” is 𝓟, which lacks the that (although it may have occurred in 𝓞).