oh hi,
if you scroll up and find the image with red circles and numbers that i posted, then i found what number 2 is - it's apparently called a 'terminator problem' and a description of it could be found for example in this blender change-log (with images). for those interested, the more technical view is in this PDF, which is also cited in that blender article. the most detailed description i could find was in the Thea forums in this thread (with images).
so, since sketchup doesn't have Subdivision modifiers (only SDS plugins, which however are a) paid and b) create solid hi-poly geometry), this is apparently an issue of shader coding and the solution lies on the shoulders of the developers...
ping, dkendig?
EDIT: and another technical PDF with the description of the problem (and apparently some solution, too)