OK, I've done some puzzling over the first of vessr's top 3 most damning examples of parallelism.
Mosiah 13:25: “they should observe to do all these things for to keep these commandments”; “they delivered them the decrees for to keep, that were ordained of the apostle and elders”: Acts 16:4. [“for to keep” is an odd word choice, and to be translated from the reformed Egyptian to capture the same terminology from the New Testament makes me a disbeliever.
The first thing that jumps out at me is the proposition that "for to keep" is an odd word choice. It occurs once in the Old Testament, once in the New Testament, and once in the Book of Mormon. Grammatically, what we have is an infinitive verb form (to X) preceded by the word "for." In modern usage, we would just use the infinitive verb form and drop the for. In fact, if you look at more modern translations of the Bible, they either replace "for to keep" with "to keep" or they rephrase the verse in a way that doesn't use "to keep" at all. You can confirm this yourself by doing a search at Biblegateway.com.
So, if we want to determine how odd the structure of "for to keep" really is, what should we look at? The "odd" part of the phrase is the placement of the word "for" directly in front of the infinitive. So, if we want to know if the construction of this phrase is odd, we need to look at the more general class of phrases that match "for [infinitive verb]."
So, let's go to LDS.org and search the scriptures for all occurrences of "for to," then count only the hits that show "for [infinitive verb]. What do we find?
Old Testament: 22
New Testament: 34
Book of Mormon: 2
I think it would be fair to conclude that the phrase "for to [verb]" is not an odd word choice in the KJV.
But, would it be an odd choice for Joseph Smith? Luckily, there is a collection of his papers online:
http://josephsmithpapers.org Let's go there and search "for to," again looking for "for [infinitive verb]." The search returns 46 hits. Here are examples of entries that fit the form we are looking for:
For to turn his hopes on high,
Collection of Sacred Hymns, 1835, Page 52
leave these sayings, with you, for to ponder in your ...verily I say unto you, my friends, I leave these sayings, with you, for to ponder in your hearts; with this
Revelation, 27–28 December 1832 [D&C 88:1–126], Page 7
5 It’s my free will for to believe:
Collection of Sacred Hymns, 1835, Page 8
For to seek eternal life,
Collection of Sacred Hymns, 1835, Page 32
& I also shall be able to purchase some goods this Spring for to & I also shall be able to purchase some goods this Spring for to make my assortment more compleet through the summer—
Order from Newel K. Whitney, 18 April 1834, Page 1
leave these sayings, with you, for to ponder in your ...verily I say unto you, my friends, I leave these sayings, with you, for to ponder in your hearts; with this
Revelation Book 2, Page 49
Revelations and Translations February 1, 1832 - November 1, 1834
last winter in order to come for to dispose of the prop last winter in order to come for to dispose of the property I had in possession, which I have been striving to...
Letter from Alvah Tippets, 20 October 1834, Page 1
goes for to p[r]ove ...be as suitable a place as any to, notice one circumstance which goes for to prove the apathy which which reigned in the civil authorities, and their...
Sidney Rigdon, Joseph Smith, et al., Petition Draft (“To the Publick”), circa 1838–1839, Page 51
we can purchase the lands with money, for to take ...a boddy, except where we can purchase the lands with money, for to take possession by conquest or the shedding of blood is something foreign...
Declaration, 21 June 1834, Page 2
I do not write this letter to entertain you with news, or for to wake you ...write this letter to entertain you with news, or for to wake you up to our dreadful condition, but that you may...
Letter from William W. Phelps, 15 December 1833, Page 1
You can visit the page and look at the rest.
So, we have a number of different types of documents, including hymns, letters, and revelations, by many different authors, that use the form "for [infinitive verb]." I consider this very strong evidence that the phrase "for to keep" was not oddly constructed when Joseph Smith lived. Based on the evidence in the Joseph Smith papers, it is not surprising at all that the phase "for to keep" appears in the Book of Mormon.
Not to harp, but this is a perfect example of why I believe reasoning from parallels should be met with a high level of skepticism.
As an aside, I also googled "for to keep." It was used a number of times, ranging from use in the 1700's to use today. Based on what I found, I think a reasonable estimate of the number of different uses of "for to keep" would be around a million, give or take 200,000.
“The ideal subject of totalitarian rule is not the convinced Nazi or the dedicated communist, but people for whom the distinction between fact and fiction, true and false, no longer exists.”
― Hannah Arendt, The Origins of Totalitarianism, 1951