The KVJ NT text was produced using the Greek text that was used by the Church throughout its history.
No it wasn't. It was translated from the only text that Erasmus could find in Western hands (plus a few supporting texts, and back translations of the Vulgate) which happened to be a late (12th Century) uncial that is now recognised to be full of errors. We have since found (mainly, actually, in old monasteries) many more reliable and less error prone copies.
If i remember correctly even common fishermen took part in writing the NT.
a) depends if you think that the traditional authorship is the correct one. I don't always.
b) they probably dictated their letters.#
It's a well-known fact that only about 5% of the ancient world, at most, were able to read and write. Maybe another 5% had some form of functional literacy (they could just about read a contract.) Especially when the church became largely Gentile, most of the people attracted to Christianity (often slaves and poorer people) would have been illiterate. That doesn't mean that they were stupid - the ancient world functioned pretty well without high literacy rates.
There would be a few educated people in most congregations (people with money, that is, to afford an education): these would be largely called upon to read the scriptures they had aloud to the congregation, and when their copies wore out, they would be responsible for copying them out in a new document. This was time-consuming, often tedious (there's marginal notes in my manuscripts that attest to this) and prone to mistakes. These were not professional scribes.
Yes, most of the mistakes are things that don't matter. But it is significant that the "mistakes" are largely mistakes of ommission: words or phrases missed out, words abbreviated with no consensus about what the abbreviation stands for, for instance. Sometimes you get someone repeating a word twice when it should only be once; and there's no agreement as to spelling. Mistakes in transcription, that is.
When you get a whole passage inserted into a text that isn't in earlier texts, that's when you start wondering if something has been deliberately altered. Especially, for instance, the 12 verses at the end of Mark, the story of the woman taken in adultery and the Johanine Comma. They are simply not to be found in earlier manuscripts.
I've a great fondness for the story of the woman taken in adultery; I like to think it did come from a genuine tradition; but it's highly unlikely to have been John who wrote it. The ending of Mark? Well, there are at least two alternatives to the one in the late manuscript. I suspect the ending was just lost. Rather careless of providence, methinks, if the preservation of scripture was what providence was about.