(I will address various parts of the OP's questions in separate posts, for readability)
Depperm's post on this question makes an effective case in favor of the credibility of the Book of Mormon witnesses, and offers a veritable treasure trove of links for further reading. I'll offer 3 distinct positive arguments for their reliability.
1. Quitting a job because of a frustrating manager
5 of the 11 witnesses never left the church (Joseph Smith Sr, Hyrum
Smith, Samuel Smith, Christian Whitmer, Peter Whitmer Jr.)
6 of the 11 witnesses did leave the church (Oliver Cowdery, David
Whitmer, Martin Harris, Jacob Whitmer, John Whitmer, Hiram Page)
2 of the 6 who left later came back (Oliver Cowdery, Martin Harris)
0 of the 11 ever denied their testimony of the Book of Mormon,
despite several having great incentive to do so
They say people don't quit jobs; they quit managers. There is a useful analogy to be drawn here to the Book of Mormon witnesses.
The 6 witnesses who left the church did so in large degree because of a falling out with Joseph Smith (this is not to say Joseph Smith was a bad manager, but he was human). The failure of the Kirtland Safety Society (KSS) during the Panic of 1837 played a role in these deteriorating relationships (I discuss the KSS in more detail in this post). So did pride--David Whitmer did not respond well when he was called upon to repent (see here).
With this in mind, let's apply the "quitting a job" analogy to consider motives/incentives.
If I quit a job because I cannot stand my manager--or perhaps more generally, I just have issues with the company's leadership--what sorts of things am I likely to say about that company down the road? You may have met people whose departure from an employer was unpleasant (I certainly have) -- ever hear them speak glowingly of the company?
Why in the world then would the witnesses who left the church--several of whom had biting comments to make about Joseph Smith--speak so forcefully, consistently, and relentlessly in favor of the authenticity of the Book of Mormon? They were the single most well-placed individuals (esp. Oliver Cowdery) to annihilate Joseph Smith's reputation and credibility by denouncing the Book of Mormon (which Joseph Smith called "the keystone of our religion") and yet, not only did none of them ever denounce the Book of Mormon, they didn't stay quiet either: they spoke actively and repeatedly in favor of the book.
This is not something we should expect--this is contrary to human nature.
It gets better. Oliver Cowdery & Martin Harris left the church, but in later years came back. They received no prominent church leadership position for doing so. No one knew better the details of the coming forth of the Book of Mormon than Joseph Smith & Oliver Cowdery--if there was a fraud, Cowdery didn't just know about it, he was in on it.
It may be worthwhile to reflect on how humiliating it would be for someone like Cowdery (or Harris) to come back, tail between their legs, and admit very publicly "I was prideful; I was wrong; I never should have left". Both had a wife/in-laws who were opposed to the church. Both came back when Brigham Young was leading the church. Brigham Young is called in Latter-day Saint folklore "the lion of the Lord"--he could chastise like none other. You didn't go admit an embarrassing failure to Brigham Young lightly.
Back to the job analogy. So now our protagonists have left the job and it was an awful break-up. They have the power to publicly destroy the company and decline to use it. Their families want them to have nothing to do with the company. And then two of them--both of whom used to be "executives" at the company--ask the hard-spoken, intimidating new CEO, if they can come back and work a rank-and-file position. Say what???
Oliver Cowdery & Martin Harris's actions--and the humiliation they put themselves through--are senseless unless they sincerely believed it was the right thing to do.
Denouncing the Book of Mormon would have resulted in praise by their neighbors (the witnesses who left the church no longer lived among church members), whereas holding to their testimonies cost them dearly. Yet all the peer pressure in the world could not get them to crack.
I propose that there is one and only one clear, viable explanation for their behavior: their testimonies were sincere. The witnesses were not perfect men, but they were men of integrity, even when it was costly to be so.
2. David Whitmer was a Theist
This argument could be made of other witnesses too--to be concise we'll just run it for David Whitmer.
The following statements of David Whitmer, the last surviving of the 3 witnesses, are worth repeating:
David Whitmer lived outside the Church for 50 years following his
excommunication—never to return but never to deny his testimony. As
the last surviving Witness, he was often interviewed—and often
misquoted. To one man who claimed that David had recanted his
testimony, he declared:
“That he may understand me now, if he did not then; and that the world
may know the truth, I wish now, standing as it were, in the very
sunset of life, and in the fear of God, once for all to make this
“That I have never at any time denied that testimony or any part
thereof, which has so long since been published with that Book, as one
of the three witnesses.”
A year before his death in Richmond, Missouri, David responded to two
encyclopedias that claimed he and the other Witnesses had denied their
testimonies of the Book of Mormon.
He declared: “I will say once more to all mankind, that I have never
at any time denied that testimony or any part thereof. I also testify
to the world, that neither Oliver Cowdery or Martin Harris ever at any
time denied their testimony. They both died affirming the truth of the
divine authenticity of the Book of Mormon.” (source)
To paraphrase Dickens, David Whitmer believed in God. This must be distinctly understood, or nothing wonderful can come of the comments I am going to relate.
For a god-fearing Christian to declare on his deathbed something that, if false, would not only stain his loved one's reputation for all time, but mark him a blasphemer in open rebellion against Almighty God, is pretty gutsy.
As David Whitmer prepared to meet his Maker, the prospect of giving an accounting to God would have loomed large next to the fading pressures of this world. Why would he testify of the Book of Mormon at this time? I suggest the simplest solution--by far--is that the prospect of standing before God having denounced what the voice of God had told Him to be true scared the hell out of him.
(I'm not cussing, I mean that quite literally)
3. Why do we trust God's witnesses, past or present
On this question's opposite number, I offered 3 reasons why Christians trust the testimony of the original apostles (or at least 11 of them--can't say I've ever heard Iscariot quoted at church):
- Their fruits
- Their willingness to die
- The witness of the Holy Spirit
I propose identical criteria here for evaluating the Book of Mormon and those who brought it to the world.
a. Their fruits
If the early leaders of The Church of Jesus Christ of Latter-day Saints did not sincerely believe their story, the meteoric rise of the faith--in spite of intense opposition--is very difficult to explain.
In this post I briefly outlined some of the costs/persecutions Joseph Smith and his family endured for sticking to their story.
The book Saints offers historians' insights on the sufferings & perseverance of the Smiths and numerous other early church members.
For a look at some of the more contemporary fruits--the actions and results of the church whose doctrine is anchored to the Book of Mormon--I recommend this sermon by a member of the church's General Relief Society Presidency, Sharon Eubank.
b. Their willingness to die
Joseph Smith and his brother Hyrum (one of the 8 witnesses) gave their lives for their faith, leaving wives and young children behind them.
A confession that it was all a hoax could have bought them their freedom. A focus on power could have given them an army to protect them from Thomas Ford (see here). They did not deny, they did not run away, and they did not engage in siege warfare with the Governor or Illinois to save their skin:
like most of the Lord’s anointed in ancient times, [Joseph] has sealed
his mission and his works with his own blood; and so has his brother
Hyrum. (statement by John Taylor who was present when they died, see here)
From Elder Jeffrey R. Holland, one of the present day apostles of The Church of Jesus Christ of Latter-day Saints:
When Joseph Smith and his brother Hyrum started for Carthage to face
what they knew would be an imminent martyrdom, Hyrum read these words
to comfort the heart of his brother:
“Thou hast been faithful; wherefore … thou shalt be made strong, even
unto the sitting down in the place which I have prepared in the
mansions of my Father.
“And now I, Moroni, bid farewell … until we shall meet before the
judgment-seat of Christ.”
A few short verses from the 12th chapter of Ether in the Book of
Mormon. Before closing the book, Hyrum turned down the corner of the
page from which he had read, marking it as part of the everlasting
testimony for which these two brothers were about to die...
In this their greatest—and last—hour of need, I ask you:
would these men blaspheme before God by continuing to fix their lives,
their honor, and their own search for eternal salvation on a book (and
by implication a church and a ministry) they had fictitiously created
out of whole cloth?
Never mind that their wives are about to be widows and their children
fatherless. Never mind that their little band of followers will yet be
“houseless, friendless and homeless” and that their children will
leave footprints of blood across frozen rivers and an untamed prairie
floor. Never mind that legions will die and other legions live
declaring in the four quarters of this earth that they know the Book
of Mormon and the Church which espouses it to be true. Disregard all
of that, and tell me whether in this hour of death these two men would
enter the presence of their Eternal Judge quoting from and finding
solace in a book which, if not the very word of God, would brand them
as imposters and charlatans until the end of time? They would not do
that! They were willing to die rather than deny the divine origin and
the eternal truthfulness of the Book of Mormon.
For 179 years this book has been examined and attacked, denied and
deconstructed, targeted and torn apart like perhaps no other book in
modern religious history—perhaps like no other book in any religious
history. And still it stands. Failed theories about its origins have
been born and parroted and have died—from Ethan Smith to Solomon
Spaulding to deranged paranoid to cunning genius. None of these
frankly pathetic answers for this book has ever withstood examination
because there is no other answer than the one Joseph gave as its young
unlearned translator. In this I stand with my own great-grandfather,
who said simply enough, “No wicked man could write such a book as
this; and no good man would write it, unless it were true and he were
commanded of God to do so.” (see here)
c. The witness of the Holy Spirit
From the final chapter of the Book of Mormon:
And when ye shall receive these things, I would exhort you that ye
would ask God, the Eternal Father, in the name of Christ, if these
things are not true; and if ye shall ask with a sincere heart, with
real intent, having faith in Christ, he will manifest the truth of it
unto you, by the power of the Holy Ghost. (Moroni 10:4)
I conclude by echoing what millions before me have affirmed: I have received that witness.