Thanks for the hard work, but I can’t seem to get the results you can.
I updated to 4.16.3, dropped in the 4.16 lightmass.exe and grabbed your scene, and I got this result with preview lighting http://i.imgur.com/ruPRq9m.png
EDIT: Seems to no have artifacts on medium or higher, but I noticed I have no bounces from the skylight with this version.