"Perhaps "invented" isn't quite the right word, but every written language has gone through a stage where a formal literary ... so, as you suggest, the period Shakespeare-KJV Bible does represent a fairly rapid establishment of what has become the standard."
I suspect that your last point in an illusion caused by the fact that most people (even many quite literate people) never read anything that predates Shakespeare & the AV. There are reasons for that - it was (and had been for a time) a period when a high level of literacy was fashionable, so that nobody was surprised at Phlip Sidney being important simultaneously in the literary and military fields of endeavour. The result was probably the greatest collection of really good writers in English that have ever been alive at one time. I think that the effects lasted will into the 17th century.
But if you do look at earlier translations of the Bible, for example those of Tyndale and Coverdale, both about a generation before Shakespeare, then ther English doesn't appear much different, which is hardly surprising as the AV took over whole chunks of them as they stood (and, even later, the Book of Common Prayer of the 1660s adopted Coverdale's translation of the Psalms in its entirety). Nor is the English of More's "Utopia" (the only work of his that I've read) noticeably different - More died 29 years before Shakespeare was born. So I don't think there was any sudden change over that period. There was, of course, a great bout of invention of new vocabulary, much of it by Shakespeare himself. But all of that comes within the flourishing of the Tudor literary age, when Henry VIII and Elizabeth I were themselves important role-models and writers of the first importance.
Looking earlier, the works of Chaucer look very different, but that's very largely a matter of orthography. If you hear them read, they sound much the same as stuff of the Shakespearean period, which would probably not be the case for Anglo-Saxon texts. That's a personal subjective judgement of how difficult I find it to understand these writers on the page and in speech - others may differ.
The work of Thomas Malory, at a date between the two, is also not far-distant from Shakespearean English. There were changes to English in the two centuries between Chaucer and Shakespeare, but they had been gradual and seem to have affected the written language more than the spoken one. Perhaps the biggest change was the fading out of inflected word endings - or was that even earlier?
If there is a period when (written) English did suddenly become much more unified and formalised, it's in the 18th century, through the work of Samuel Johnson and his contemporaries, which brought the idea of "correct" spelling into being. Before then people, even highly literate people, felt free to spell a word as they felt suitable to its context. Think how many ways Shagsberd / Shakeshaft had of spelling his own surname!
-- Mike Stevens, narrowboat Felis Catus II Web site www.mike-stevens.co.uk No man is an island. So is Man.