There appear to be at least three distinct (unstated) questions here that need to be addressed first before we can answer the stated question.
What books, if any, are "missing"?
This really leads to a discussion of canonicity, which is outside the scope of the site. However, I'm not aware of any extrabiblical writings that would pass the tests of canonicity - authority, consistency and dependability. Most of the writings that are normally held up in this fashion, such as the Gospel of Thomas, are of unknown or spurious authorship, late in date (as compared to the earliest known manuscripts of the canonical writings) or inconsistent in message.
This site also talks about canonicity in the context of these supposed "lost" books. According to the linked article, the primary tests for (NT) canonicity used by the early church were:
- Was the book/letter written by an apostle or one of his associates?
- Did it agree with other recognized writings?
- Was it used widely by the early church?
(Just so nobody thought that my explanation of canonicity came from thin air... :) )
What changes, if any, have occurred to the scriptures, and how do they affect our understanding?
We don't have the original autographs of any canonical book. Modern culture has spoiled us a bit - within the past 50 years or so, multiple technologies have been invented that permit perfect or near-perfect copying quite easily. For thousands of years, the only way to make a copy of something was to either write it out by hand, or (following the invention of the printing press) engrave or typeset it, again by hand. Copyist mistakes are clearly a possibility, if not likelihood. However, they also created various ways to ensure that such mistakes did not creep in. Having multiple copies of the various books, we can also trace back through different branches of copyists and determine with a fairly high degree of accuracy what the originals probably actually said vs. what has been introduced or mangled later by a copyist.
In addition, making copies was the only way to spread the manuscripts to a wider audience. They couldn't toss it on a flatbed scanner or run down to Kinko's to photocopy. They didn't have a pervasive Internet to spread information world-wide in seconds.
That said, even among manuscripts that differ, they are still surprisingly consistent. Certainly, if a majority (or significant minority) of copyists were intent upon injecting or modifying meaning, there would be many more differences and they would be more remarkable.
What "lapses in memory" may have caused supposed errors in the autographs?
I'm not exactly sure what you were referring to here - perhaps differing accounts of the same events by the gospel writers? Dr. Craig Blomberg answers this fairly thoroughly in The Case for Christ, but the core of his argument is threefold:
- Literary purpose - unlike our modern concept of a biography, ancient writers did not necessarily write about events in chronological order, nor would they have recorded every saying word-for-word. Instead, they would have focused upon preserving meaning and drawing the reader's attention to relationships between events. Certainly, at the time of their writing, there would have been enough individuals still around who remembered the actual event and could have objected to any errors.
- Theological purpose - the authors of the gospels each wrote with a particular goal or an aspect of Christ's life and ministry in focus. Therefore, their writings emphasized different things.
- Many differences could be explained by the simple fact that eyewitness accounts will vary, even if all of them saw the exact same event. Some would catch certain pieces, while others might have skipped those but latched onto a different piece. Lee Strobel (the author of the book previously mentioned) makes the analogy to a reporter who also talks with other reporters at the same event, or records the interview or press conference on tape, to ensure that his notes are accurate.
So, to the stated question:
Is Jesus making a "plain" statement, and if so, what is it? And how do our answers to the unstated questions inform us?
Stating this a different way, what is the meaning of "pass away" in these verses? Cease to exist? Cease to apply? Are we talking physical destruction, spiritual loss (application) or something else?
The Matthew 5:18 passage is in a slightly different sense from the other two, both in its object and duration. In this case, the object is the Law/Prophets, and its duration is "until fulfilled". In other words, the Law/Prophets would be fulfilled - completed, satisfied, full - within the scope of Earth's existence. In fact, it would be within the scope of His ministry. Whether or not they continued to exist beyond that, it's really not covered within the scope of the statement.
Regarding the other two, if Jesus meant for these to be taken as literal or plain statements of fact, it's difficult to derive from the text.
The physical sense is extremely difficult to apply here - clearly, as mentioned above, we've lost all original autographs. Stuff breaks down - it gets old, fragile, lost, damaged, vandalized. When Jesus read from the scroll of Isaiah, He clearly would have known that he did not hold the original autograph of that book - He was in a bywater synagogue, more than 400 years after its writing. The original autograph probably didn't even exist any longer, and certainly not in Nazareth.
Linguistically, we're also in a very different place. Very few of us speak fluent Hebrew, Aramaic or Koine Greek. Equally few of us speak Latin, the common language of the Church for about 1,000 years. We have multiple translations to and from multiple languages. It probably shocks some, but Jesus didn't speak in King James English. So if He meant the literal words that He spoke, they too are long, long gone. (And as Jon so ably noted while I was typing all of this, in many cases the gospel writers didn't even record everything He said.)
Finally, the overall context of the passage does not help in this regard either. Jesus was telling a parable to His disciples, and this statement in particular seems to be within that context; these words - the prophetic parable - would be fulfilled. It was not meant to cover the scope of His entire ministry.