With respect, I would submit that, if you do your research from the documented history of the English language then you would see that it was, in fact, everyday language in 1611. The Merriam Webster dictionary states that for most speakers of southern British English, thou had fallen out of everyday use, even in familiar speech,
by sometime around 1650. And that's just in the south of England. That may not mean much to you, but as an Englishman, that's very significant. What's it's saying is that in the 'cultured south' - London etc - the use of these and thous was going out of fashion by around 1650. But the rest of the country would still have used those forms in everyday speech. In fact, thees and thous were the standard usage in informal settings which was one reason the singular went out of fashion. It was seen as too 'familiar' and not polite enough. So, polite society stopped using thees and thous forty years after the KJV was published. But the rest of the country used them for a lot longer. In fact, there are still a few places (they would be seen as the UK's equivalent of 'redneck country' I guess!) where the gentrifying influence of London still hasn't fully reached even in 2017 and you'll still hear people addressing each other as 'tha' (thou). So I can tell you with some authority that to say 'They were not using it in every day language in 1611' is simply not the case. Interestingly, the places where you'll see that urban myth perpetuated are almost exclusively Christian. (Fake news clearly isn't a new phenomenon!)
With regards my 'innovation' - the King James Easy Read edition from Whittaker House uses the same method for distinguishing pronouns and it works perfectly well. And if we raised 'but there might be other ways of doing it' to every new thing that was proposed then nothing would ever get done. Which bring me back, again, to my initial question. I think that that question has now been more than adequately answered though! The answer was NEVER!