Well, I think you know that I don't buy into your premise--if there is such a premise in your question--that inauthentic Greek is NECESSARILY a bad thing. Simplified Greek, which is by definition inauthentic Greek and is as a practical matter bad Greek, has a role--an underutilized role in my opinion--in learning the language. At some point, less simplified Greek, and therefore more authentic and better Greek comes to be an advantage. But I think even radically simplified (and therefore inauthentic and bad) Greek has a role in at least the early stages of learning. Of course I think this, because nobody's adapted Greek is more simplified, inauthentic and bad than some of my more basic "leveled" Greek readings.viewtopic.php?f=28&t=22003
I should say I THINK some of my most basic readings, which of course make no attempt to be authentic or good Greek, are useful for beginning to intermediate learners. I don't know for sure if they are. The only way that anyone will ever know if they are useful or not is if people try them, but even then there will not be a control group of people who spent the same amount of time reading something else. If somebody thinks that by reading my Greek their Greek fluency will be HARMED, then of course they should stop reading, but I've never thought it possible that one's Greek CAN be harmed by any resource.
Having said this, one can evaluate the various text books on the standard you have proposed. JACT does make an attempt, I think, to be more authentic than Athenaze, which really means that it is harder, (and arguably less useful) for beginners. (It was for me.) The exercises in the Greek Ollendorf are not of course authentic Greek because they are designed to be short and simple and easy to assimilate, which they wonderfully are, particularly when heard read aloud. The made up sentences of Mounce are a little less authentic than those of Machen. Of the two marvelous adapters of Xenophon, Phillpotts is more authentic, and in my opinion, less useful, than his colleague Jerram. Rouse is more authentic than Morrice. Rico's text is of course not very authentic and is arguably the best single resource every produced for learning Ancient Greek.
You get the point. I know many, maybe most, will disagree, that real Greek is always better than adapted Greek and therefore the most authentically adapted Greek is always to be preferred. I respect the views of such people, but I disagree, and I think when it comes to learning Greek, purists are not made, but born. What is good about public forums is that you get all views, or should I say all tastes, aired.