These are all questions that relate to the complex discipline called Textual Criticism - the study of the texts and manuscripts upon which our translations are based.
I would commend the first half of John Piper's message on John 7:53-8:11 in which he addresses this complex subject very well.
Here are a few notes:
- The New Testament was originally written in Greek
- The first Greek New Testament wasn't published until 1516 by Erasmus
- For 1500 years, manuscripts were handwritten copies
- None of the original manuscripts exist but multiple copies are available.
- There were four kinds of copies: uncials (large caps), minuscules (small caps), papyri, and lectionaries (sections read in worship)
- There are 322 uncial texts, 2907 minuscule texts, 2445 lectionary portions and 127 papyri for a total of 5,801 manuscripts - all handwritten copies
- No ancient book comes close to this level of manuscript evidence (for instance, Julius Caesar's Gallic Wars, written between 58-50 BC only has 10 copies)
- The copies do not all agree but no major doctrine is threatened by the differences
- The number of manuscripts increases the possibilities of differences but it also increases the means of correcting errors
- The margin of doubt, according to FF Bruce, in recovering the exact original wording is very, very small