Rondec: here's an update on what I've found. For comparison purposes I loaded the original ~156MB pixel shift DNG of the above scene back into the K-3II, and developed it into both TIFF and JPEG in camera - first time I've done this.
Both showed the same scene as in the above left image, no visible artifacts. The raw development options menu does actually have a selector for pixel shift on or off (it's on by default when you load the options up) but switching it to off appears to have no effect in the camera; there are still no artifacts in either case in the output file. But take the DNG into DCU5 and in the laboratory tabs, the default 'camera setting' view shows no artifacts - which you can switch off, and then it
does make the difference shown above.
I can only conclude that for pixel shift in the K-3II;
- motion correction must be on permanently (there being no switch options),
- developing the DNG into TIFF/JPEG in camera shows no artifacts, so that the camera appears to be processing the four images down into one and then removing artifacts,
- artifacts do exist in the original DNG file because in DCU5 it's possible to turn the motion correction off, and see them clearly.
Fascinating discussion! And of course, keep on with the great images that you post here.