As a K100D owner I have been reading for a couple of years how dismayed the newer camera owners are with the inability to turn off long exposure NR. From what I read it is the CMOS sensor cameras that lack the option because they are noisy for long exposures. I know an accessory that will fix the problem! Just buy a used K100D for a few hundred bucks.
It is sometimes better to have a separate camera for special jobs like night photography than force a camera that is unsuitable. Used Pentax DSLRs are so cheap these days, less then some people spend on a one lens.
When I do Star Trails I set the exposure to 30 seconds, NR off, and Continuous Mode. I attach the simple Pentax mechanical remote (basically just a locking button), connect a external supply, and let it run for hours. When done I put on the lens cap and take a few dark frames of 30 sec.
The German program StarTrails does the stacking and the dark subtraction.
www.Startrails.de-Home