There is of course probably a much simpler explanation to all of this: mistakes. At least, "mistakes" from the point of view of writing. This was, after all, a dictated text, and if any of you have ever attempted to take dictation (or give it), you know how difficult it can be to fit spoken discourse into the strictures of writing, with its demand for clear-cut clauses and obedience to the signposts of punctuation.
The examples that Scorndog has given are two. One ("teasings") is irrelevant because a quick search on GoogleBooks reveals that that word was not only current in the 19th century but was used as recently as 2014. The other is not really that problematic either, though. Scorndog cites "but if" with the meaning of "unless." But does it really have that meaning in the passage?
For the natural man is an enemy to God
and has been from the fall of Adam, and will be, forever and ever
but if he yieldeth to the enticings of the Holy Spirit
and putteth off the natural man
and becometh a saint
through the atonement of Christ the Lord
and becometh as a child,
submissive, meek, humble, patient, full of love,
willing to submit to all things which the Lord seeth fit to inflict upon him,
even as a child doth submit to his father.
And moreover, I say unto you
that the time shall come when the knowledge of a Savior shall spread
throughout every nation, kindred, tongue, and people
(Mosiah 3:19-20, ed. Skousen)
Skousen reads this text from the perspective of the post-1920 Book of Mormon text, when James E. Talmage changed "but if he yieldeth" to "unless he yields." Skousen thus assumes that Talmage's emendation of "but if" is identical to Joseph Smith's meaning. He then goes to the OED, finds an entry for "but if" to be "unless" with a reference to Spencer and a few others, and then concludes that this is evidence of Elizabethan diction.
Suppose, though, that Talmage is trying to impose the kind of transparent syntactical coherence we expect in written discourse but which routinely absent from spoken discourse. If you read that sentence out loud, keeping the "but if," then it looks like Joseph Smith just started a sentence that got away from him. When this occurs in writing, it's called aposiopesis. Here is an example contemporary to Joseph Smith of a literary work with aposiopesis:
Mary Pinckney, in The Young Carolinians, 1818 wrote:I hope my letter safe in the British consul's hand. If the consul is the gentleman I suppose him to be, he married one of my aunts. It is long since death dissolved the tie that bound him to our family, yet I have heard my good aunt Woodberry say, he was a most tender husband, and will no doubt prove faithful to his wife's memory. How fortunate that I discovered his name. Surely he will not refuse to advance our ransom, well knowing our ability to repay the debt; but if I should be mistaken—many bear the same name. O, how I languish for certainty. Achmet promised I should be sent for; but I may be forgotten. Alas! I was compelled to depart without having it in my power to inform poor Ellinor that her St. Julien is a captive. How restless—how impatient—how anxious is my mind? Should Mustapha persist in sending her to the dey before her ransom is obtained, what will become of my hapless sister?
In what I have highlighted, the "but if" could just as easily be, "unless," but notice that the thought just breaks off, and the break is marked by the dash. Punctuation marks the aposiopesis. The Book of Mormon text was underpunctuated, you might say, and even Skousen adds punctuation in his edition. But if we repunctuated Mosiah 3:19 with a dash, it would look like aposiopesis.
As apologists like to point out, Joseph Smith weren't no ejiicated feller, and he was dictating his text by all accounts, so it would be no surprise to find features of oral discourse in that transcribed text. It would be surprising if they
weren't there. What is surprising to me, though, is how the line between written and spoken is constantly ignored in these discussions, but I stress again: nobody speaks the way that they write. It's a different discourse. But when strings of spoken speech become written text, they end up looking like mistakes. Later editors, including Joseph Smith himself as you all know, ironed out those "mistakes," but Skousen, as far as I know, holds to the theory that the first revealed text is by definition without mistake and therefore he sees the diction of high Elizabethan poetry where he can. Has he explained anywhere why that is so?
Lastly, this is the ONLY example in the book of "but if" that could be construed as "unless," even though the collocation "but if" occurs more than two dozen times in the Book of Mormon, all with the same meaning as it has today, and all of them therefore consistent in their common meaning. There is only deviation from that consistency, but ne occurrence is not at all that significant in 500 pages of dictated text—especially when the Book of Mormon uses "unless" nearly ten times. Why would there be just this one instance of Elizabethan usage—from high poetry of all places—when "unless" is used everywhere else? If "but if" occurred every place that we would expect "unless," then there might be something linguistically unusually, but that's not what we have.
If you just read this aloud to yourself and treat it as an oral text, it seems pretty clear that the sentence—with all of those connective clauses and some subordinate clauses piled one on top of the other—just got away from Joseph Smith as he dictated. The "if" clause got so large, and he got lost before he could get to the "then" clause, to put it simply. That is extremely common in spoken discourse with extended protases ("if" clauses), and as an aside, ellipsis of a "then" clause is the origin of several syntactical features of Biblical Hebrew, if I may be so kind as to throw a bone to the apologists, as well as other languages (e.g. Homeric Greek, another written language shot through with markers of oral discourse).
Perhaps someone could point me to the "100+" examples of Elizabethan diction referenced on this thread, because all of the examples that Skousen lists in his Yale edition of the text could just as easily be explained as the kind of phenomena endemic to oral discourse that seem like they are "mistakes" when put into written form.
"As to any slivers of light or any particles of darkness of the past, we forget about them."
—B. Redd McConkie