It was the late 1970s when, in Bible college and seminary, we sat through lectures from at least two older Jewish-Christian scholars (that is, they had been raised and educated in Judaism and had met their Messiah, Jesus Christ, later) who spoke on the subject of how manuscripts were so meticulously copied by scribes before there were any printing presses. These lecturers focused on the rules and regulations binding on Old Testament scribes before Christ’s day. The rules were very strict.
These men spoke of how unintentional errors were corrected when copying manuscripts. According to those old Jewish men, when certain errors were made that could not be corrected, that manuscript was given a ceremonial burial as if people were being buried. The manuscript was, in affect, declared to be dead.
Those men both said that the manuscripts found among the Dead Sea Scrolls, found in a BURIAL place, would be examples of these. They are not useless to history or archeology, but they should not be used as a sure authority for deciding the authenticity or accuracy of any given passage of Scripture, because the likelihood of error in them is great.
The Jews of Jesus’ day, I believe, had preserved copies of the original books of the Hebrew Bible. Jesus read them in the synagogues and quoted from them without any criticism of them or attempt to correct any of them. And they were, in fact, HIS words.
Firstly, the number of difference manuscripts represented is nearly one thousand. This was an entire library, unless the Essenes made a ton of mistakes. ;-)
Second, These are not like other libraries that have been found, because these were buried for the purpose of being found in the later times. They have been preserved. Ironically, the mistakes you mention, are the opposite of why these were buried, at least IMO. Just as surely as you copy a document, you will make mistakes; and just as surely as the next person comes along and copies your mistakes, he will try to correct them; and just as surely as he tries to correct them, he will make new mistakes. The next person will come along and try to correct him, and before you know it, the document is a mass of corruption, whether deliberate or not. But no document can ever escape these basic distortions and corruptions, except if buried to come forth in its purity at a later time.
And so the DSS, we find a library buried and sealed in jars. The DSS were first written on nice, newly prepared leather, then rolled up and wrapped carefully, and covered with linen; then the linen was covered with pitch, Then they were put in specially made cylindrical jars, sealed with lead and pitch on top with caps that fit on tightly. Then they were arranged neatly in a cave and covered with nice, dry sand so there would be no corruption; everything was hermetically sealed. Then the cave was cemented up so you couldn't see a thing; you're not coming back next week to get them. That's not merely a guess, because the documents themselves tell us why they were buried in this way and what the owners had in mind.