If such may have been the fact, that a part of the Ten Tribes came over to America, in the way we have supposed, leaving the cold regions of Assareth behind them in quest of a milder climate, it would be natural to look for tokens of the presence of Jews of some sort, along countries adjacent to the Atlantic. In order to this, we shall here make an extract from an able work: written exclusively on the subject of the Ten Tribes having come from Asia by the way of Bherings Strait, by the Rev. Ethan Smith, Pultney, Vt., who relates as follows: "Joseph Merrick, Esq., a highly respectable character in the church at Pittsfield, gave the following account: That in 1815, he was leveling some ground under and near an old wood shed, standing on a place of his, situated on (Indian Hill)... [Joseph then discusses the supposed phylacteries found among Amerindians, citing View of the Hebrews p. 220, 223.]It would be quite silly to quote from a book that you used as source material; you would be immediately exposed as a fraud, especially given the fact that numerous people had made it their life's mission to expose you in any way they could. Oliver Cowdery, who knew Ethan Smith, would likely have outed the Prophet's plagiarism, given that besides Joseph Smith no one knew more about how the Book of Mormon was translated. After his excommunication in 1838, it would have been simple to bring down the Prophet's forgery. Yet Oliver Cowdery never denied his testimony of the Book of Mormon, and later came to Utah and was re-baptized. This would very contradictory if Cowdery knew the Book of Mormon was a fraud.
B.H. Roberts, who was a member of the First Council of Seventy and perhaps the Church's greatest theologian and historian, wrote a collection of essays known as Studies of the Book of Mormon. In them, he plays the role of devil's advocate, looking at every way an outsider could attack the Book of Mormon as being non-historical, or viewed as the creation of Joseph Smith. This volume was never meant for publication, and was sent to the First Presidency for their viewing.
Some have alleged that Roberts lost his testimony of the Book of Mormon because of the things stated in the essays. There are two glaring problems with this theory. Roberts wrote a letter along with the manuscript that said the following:
"In writing out this my report to you of those studies, I have written it from the viewpoint of an open mind, investigating the facts of the Book of Mormon origin and authorship. Let me say once and for all, so as to avoid what might otherwise call for repeated explanation, that what is herein set forth does not represent any conclusions of mine. The report herewith submitted is what it purports to be, namely a 'study of Book of Mormon origins,' for the information of those who ought to know everything about it pro et con, as well as that which has been produced against it. I do not say my conclusions for they are undrawn. It may be of great importance since it represents what may be used by some opponent in criticism of the Book of Mormon. I am taking the position that our faith is not only unshaken but unshakable in the Book of Mormon, and therefore we can look without fear upon all that can be said against it."Close quote. That does not sound like a person who has lost his faith; it is the testimony of an honest scholar. Also, keep in mind that B.H. Roberts was a man known for his integrity and defending his beliefs, even when it could make for a very uncomfortable situation; he denounced the 1890 Manifesto as revelation, and almost had to resign as a general authority because he did not want the Church infringing on his right to run for public office. However he publicly testified about the truthfulness of the Book of Mormon until his death. If he was a skeptic, there is no evidence for it.
Second, Roberts magnum opus The Truth, The Way, The Life: An Elementary Treatise on Theology, written in 1930 (after Studies of the Book of Mormon) relies heavily on the Book of Mormon. Roberts worked in vain to get the Church to publish it, but it was never published due to Roberts dispute with Joseph Fielding Smith over evolution. This would be inconsistent with Roberts being a skeptic about the authenticity of the Book of Mormon.
On Runnells last point, it should be remembered that scripture should be taken in light of what the Church teaches, not the other way around as Richard Swinburne pointed out in a debate with Bart Ehrman. Doing so frees us from St. Peter's warning that the scriptures are not for private interpretation (2 Peter 1:20), and also that God gives prophets, not books, to lead his people (God sent Moses, not the ten commandments to lead his people and instruct them.)
Also, while the Book of Mormon does makes reference to the Father and the Son being one (Ether 3:14), and Jesus being God himself (Book of Mormon cover page, Mosiah 15 1-4), there is no mention of God being manifest in three divine persons, but being one God. Also, if the Book of Mormon were Trinitarian, Jesus would announce himself as being the Son only when appearing to the Brother of Jared. Yet he calls himself "The Father and the Son." The text also mentions nothing, even in the alleged contexts, of God being one substance as common in Trinitarian creeds. The Book of Mormon simply is not Trinitarian.
Further Reading: Exploring Mormon Thought: The Attributes of God by Blake Ostler, Studies of the Book of Mormon by B.H. Roberts, Rough Stone Rolling by Richard Bushman