<Who and how can set the mirror stop/do necesary fine tunings?>
Nikon Repair would make any required adjustments. BUT - don't blame the gear without exhausting other sources of soft images.
One thing to remember with the D800 is that a 100% crop showing 1000 pixels or so is much smaller and more magnified than a 100% crop on earlier cameras. On the D800, 1000 pixels is just 1/7 of the image while on the D200/D300 is is 1/4 the image - near the equivalent of a 200% view.
Sharp images require excellent technique. Good technique takes practice - both with focus and with stability. Start on a tripod with Live View, move to a tripod with Mirror Up and learn what the gear can do. Keep your shutter speeds at 1/1000 sec or more and f/8. Then start working on shallower apertures with fast shutter speeds. Don't expect to get quality test images in marginal situations such as low light, slow shutter speeds, etc.