I'm not sure if you're looking for a general explanation or for responses to these specific "prophecies." I'll respond the the first two and then wait for your answer before delving in deeper. Before I do, though, I'd just like to say that I agree with Jane's
post #62, at least to the extent that I would suggest you not spend so much time trying to disprove Mormonism and a little bit more time studying its doctrines and teachings. You will always be able to find issues for which there seems to be no good answers, but I really think that this would be true regardless of which religion you're studying. I've been a Mormon my entire life, and I can still find reasons to doubt. I'm not saying that you should just ignore your doubts, just that you should put them into perspective. For every reason I can find to disbelieve Mormonism, I can find ten to believe it. I can let the one reason consume me or I can embrace the ten.
Don't get me wrong. I think the pendulum can (and often does) swing too far the other direction. I know of two women in my ward, both about my age, who have said to me, in essence, "I don't know. I've never really given it very much though -- I mean about whether the Church is true or not. My parents always told me it was. And I figure if it was good enough for them, it's good enough for me." It was all I could do not to lay into these women and ask them what on earth they were using their brains for, other than to decide what to fix for dinner. There are far too many Mormons who take that position, just as there are far too many non-Mormons who will stop at nothing to prove that Joseph Smith was a fraud and that Mormonism is a religion designed for idiots. Nobody should ever belong to a religion because their parents did.
Having said that, here are my answers to the first two "propheces" that supposedly "failed":
Critics of the Church have mentioned that Joseph Smith "prophesied" that "God had revealed to him that the coming of Christ would be within 56 years." Interestingly, we have no actual transcript of any sermon containing such a prophecy. There are references to a sermon in which Joseph was said to have mentioned "the coming of the Lord, which was nigh -- even fifty-six years should wind up the scene." Remember that we're getting this information second- and third-hand from several individuals whose recollections of the sermon have been combined into a single narrative and not from any original document.
But let's look at the events which led Joseph to make a statement that apparently was taken by at least some members of the Church to be a prophecy. Joseph had previously prayed to God, asking when the Second Coming of Christ would be. In his own words, we read: "I was once praying very earnestly to know the time of the coming of the Son of Man, when I heard a voice repeat the following: 'Joseph, my son, if thou livest until thou art eighty-five years old, thou shalt see the face of the Son of Man; therefore let this suffice, and trouble me no more on this matter.'" He continued, "I was left to draw my own conclusion concerning this and I took the liberty to conclude that
if I did live till that time, He would make His appearance." Roughly a month later, he stated, "Jesus Christ never did reveal
to any man the precise time that he would come."
It is entirely possible that Joseph believed that "56 years should wind up the scene." But, when analyzed with his other statements in mind, I don't believe he was making a prophecy at all. If I have invited company to come for dinner at 6:00, at 5:45, I might say to my husband, "Dave and Carol should be here in fifteen minutes." That would be my best guess, based on what time I told them to arrive and what I know about their punctuality. But if they didn't show up until 6:15, both my husband and I would just conclude that they were late.
In The Doctrine and Covenants (Section 84) we read, "For verily this generation shall not all pass away until an house shall be built until the Lord...." I'm assuming that's the "prophecy" you're thinking of. I can't help but wonder what the same individuals who call this a "false prophecy" have to say about Jesus Christ's words as recorded in Matthew 24:34. One of His disciples had asked Him what the signs of His return and of the end of the world would be. Jesus mentioned a number of signs. In fact, much of the chapter includes His enumeration of what they would be. Then, finally, He says, "Verily I say unto you, This generation shall not pass, till all these things be fulfilled."
Whoa! Did Jesus Christ falsely prophesy of His Second Coming? If so, then Joseph Smith's not in bad company, is he, when he said that a temple would be built in "this generation."
I think of Section 84 of the D&C as being more of a commandment than a prophesy. As things turned out, the Latter-day Saints were forcibly driven from their homes before a temple was built. In D&C 124, the Lord says, "Verily, verily, I say unto you, that when I give a commandment to any of the sons of men to do a work unto my name, and those sons of men go with all their might and with all they have to perform that work, and cease not their diligence, and their enemies come upon them and hinder them from performing that work, behold, it behooveth me to require that work no more at the hands of those sons of men, but to accept of their offerings."
I believe the Lord would have been very pleased had a temple been completed earlier than it was, and that He wanted the Saints to show their devotion to Him by proceeding as if they fully intended to build it. But I don't see the fact that they were unable to as a "failed prophecy."
Just as a final note, if you'd like me to, I can point out some "false prophecies" by a couple of Old Testament prophets, too.