Well..........I contacted a couple of gurus and considering ALL the advice, both from them, and what I have seen here (& at DPreview), the D600 will go back to Nikon with an even switch for it and the "kit" lens as soon as another kit version arrives at Fort Worth Camera.
I expect I am fortunate that this glitch happened right out of the box, and to the camera salesman at the counter, and he vividly recalls how embarrassing this was with the store full of folks looking at the then brand new D600. So, no problem with the Nikon tech being unable to replicate the problem. The diopter refusing to adjust is just icing on that cake, and hence my decision to switch bodies (and lens). I was reluctant, because mine has not had dust/oil issues and the lens was fine, with no need of finetuning (which is hard on a zoom).
Fort Worth Camera will then check and fine tune as necessary ALL my lenses. Afterward, I'll be out shooting again, likely as happily as before.