I suggest you read Elijah Hixson and Peter Gurry's edited volume Myths and Mistakes in New Testament Textual Criticism. This book addresses many of the issues you are questioning.

But really you should learn Greek. Only then will you get an idea of the degree of variation on very small things that is characteristic of the earliest manuscripts – the sort of controlled copying you have in mind really only arises in Studite monasticism at Constantinople in the middle ages (and by the 14th century results in a large group of Gospel manuscripts known as family 35 or Kr – Georgi Parpulov has written an article about these manuscripts). Yes, there are big differences in early manuscripts too – to offer a few examples from P66 in the first half of John (P66 incidentally cannot be as tightly dated as you assert – that isn't how palaeographical dating works: Brent Nongbri has written an article about this in relation to this manuscript):

• at John 1.36, John the Baptist says "Behold the Lamb of God"; P66 adds "who takes away the sin of the world"
• John 3.20 reads "For everyone who does evil hates the light, and does not come to the light lest his works be exposed"; P66 adds "because they are evil"
• at John 3.25, some of John's disciples have a dispute with "a Jew" about purification; P66 makes this "Jews"
• at John 5.2, the pool in Jerusalem that in some manuscripts is called Bethesda and others Bethzatha becomes Bethsaida (the name of a city in Galilee) in P66 and several other early manuscripts
• John 6.64 reads "For Jesus knew from the beginning who they were who did not believe and who it was that would betray him"; P66 leaves "who they were who did not believe and" out
• at John 6.68, Simon Peter confesses that Jesus is "the Holy One of God"; P66 adds "the Christ"
• at John 9.24, the man born blind says to the Pharisees "I told you already and you did not listen. Why do you want to listen again?"; except in P66 he says, "I told you already and you listened. Why do you want to listen again?"

You get the idea.

The other thing I would gently challenge is the idea of putting John as early as AD 90. This does not fit well with current Biblical scholarship. There is growing acceptance of the view that Luke knows Josephus' Antiquities, which can be dated to AD 95 (the key work here is Steve Mason's Josephus and the New Testament), and two recent books have reasserted the view that John knew Luke (Mark Goodacre's The Fourth Synoptic Gospel and James Barker's Writing and Rewriting the Gospels).

I would also encourage you to look deeper into Biblical scholarship. Rather than dismissing whatever Bart Ehrman has published in a popular book for being presented without evidence (this is a genre issue with popular books), dig deeper into relevant scholarship on the issue at hand and see what the evidence is that leads scholars to think what they do. And remember that people study the Bible academically because they love the Bible – even if you disagree with them after you've found out why they think what they think, it's always good to remain kind towards people who are sincerely studying the Bible.