What did Alma know, and when did he know it?

At the very beginning of Nephite history, Nephi himself prophesied that Jesus would come 600 years from the time Lehi left Jerusalem:

"Yea, even six hundred years from the time that my father left Jerusalem, a prophet would the Lord God raise up among the Jews -- even a Messiah, or, in other words, a Savior of the world" (1 Ne. 10:4).

"And behold he cometh, according to the words of the angel, in six hundred years from the time my father left Jerusalem" (1 Ne. 19:8).

"For according to the words of the prophets, the Messiah cometh in six hundred years from the time that my father left Jerusalem; and according to the words of the prophets, and also the word of the angel of God, his name shall be Jesus Christ, the Son of God" (2 Ne. 25:29).

However, when Alma the Younger is preaching an Ammonihah circa 82 BC, he appears to be unaware of this prophecy, as he says they "know not how soon" Christ will come:

And now we only wait to hear the joyful news declared unto us by the mouth of angels, of his coming; for the time cometh, we know not how soon. Would to God that it might be in my day; but let it be sooner or later, in it I will rejoice (Alma 13:25).

The skeptical explanation for this discrepancy goes something like this: According to the well-established theory of Mosiah priority, Joseph first dictated the lost 116 pages, then continued dictating from Mosiah 3 (now Mosiah 1, as the first two chapters were lost) to the end of the book, and finally went back and dictated from 1 Nephi to Words of Mormon. By the time he dictated the "small plates" books, Joseph Smith no longer remembered many of the details from the lost 116 pages, but he was worried that the pages might resurface at any time and didn't want there to be any discrepancies. That is why the "small plates" books give so few historical details and name so few of the characters. To pad out this section and make up for the lack of any detailed history, Joseph Smith filled the small plates with lots of prophecies, including Nephi's detailed visions of the future. The problem was that he didn't know he would later be creating these visions for Nephi at the time he dictated "large plates" books like Alma, and so the characters in that part of the book are inexplicably ignorant of what Nephi prophesied.

The only believing explanation I have encountered is simply that, for whatever reason, the contents of the small plates were just not common knowledge among the later Nephites. In support of this, we have Mormon circa AD 385 speaking of the small plates as if they were some obscure document buried in the archives, which he had not known about before:

And now, I speak somewhat concerning that which I have written; for after I had made an abridgment from the plates of Nephi, down to the reign of this king Benjamin, of whom Amaleki spake, I searched among the records which had been delivered into my hands, and I found these plates, which contained this small account of the prophets, from Jacob down to the reign of this king Benjamin, and also many of the words of Nephi (W of M v. 3).

So it appears that, whatever the reason may be, Alma the Younger did not have access to the small plates. One problem with this assumption, though, is that in his words to his son Helaman circa 74 BC he appears to quote from them directly:

Yea, methought I saw, even as our father Lehi saw, God sitting upon his throne, surrounded with numberless concourses of angels, in the attitude of singing and praising their God; yea, and my soul did long to be there (Alma 36:22).

The passage in boldface is a 20-word verbatim quote from the small plates:

And being thus overcome with the Spirit, he was carried away in a vision, even that he saw the heavens open, and he thought he saw God sitting upon his throne, surrounded with numberless concourses of angels in the attitude of singing and praising their God (1 Ne. 1:8).

If we want to maintain that Alma really didn't have the small plates, I guess the explanation must be that this passage was also in the lost 116 pages.

For the believer, the large plates, believed to have started with a Book of Lehi, would surely have contained an account of Lehi's Jerusalem vision, and Nephi's own account of that vision would likely have quoted some of his father's language directly. Alma wasn't quoting Nephi; rather, both Alma and Nephi were quoting Lehi.

For the skeptic, it is unlikely that Alma's quote would match the language of the lost 116 pages exactly, since Joseph Smith didn't have those pages when he wrote Alma. But when he was writing the replacement text ("small plates"), Smith referred to Alma and made sure to make Lehi say what Alma had said he'd said. If Smith was doing this, though, he wasn't very consistent about it. For example, Alma claimed that Lehi and Nephi called the ball Liahona (Alma 37:38), but Smith apparently forgot to include that word in the replacement text.

I've been familiar with the above arguments for some time. However, not until my umpteenth rereading of Alma just today did I notice another highly relevant passage. This is Alma addressing his son Corianton circa 74 BC

And now, my son, this was the ministry unto which ye were called, to declare these glad tidings unto this people, to prepare their minds; or rather that salvation might come unto them, that they may prepare the minds of their children to hear the word at the time of his coming (Alma 39:16).

Here Alma starts to say that Corianton is to prepare the minds of the people to hear Christ when he comes, but then he corrects himself and says that no, actually, it's to help them prepare their children to hear Christ when he comes. This implies that Alma knows Christ is not coming soon enough for people who are adults in 74 BC to hear him themselves but is coming soon enough for those people's children to hear him. In other words, he appears to know the date of Christ's coming with considerable precision, and this is in stark contrast to what he had said just eight years earlier -- when he said "we know not how soon" Christ will come and seemed to entertain the possibility that it would be in his lifetime.

So in 82 BC, Alma doesn't know when Christ is coming -- but then in 74 BC he (1) appears to quotes verbatim from 1 Nephi and (2) suddenly does know when Christ is coming. This strongly suggests to me that at some point between those two dates, Alma gains access to the small plates and becomes familiar with their contents.I'm going to have to go back through the relevant portion of the Book of Mormon with that hypothesis in mind and see if I can find any hints of exactly when and how that might have happened.