Here is a video showing how it should look (not mine):
https://www.youtube.com/watch?v=DKjquLIe-d4

Moving to the todo list for now.

I think it's related:
Luxrender currently has option for custom ground color, really nice feature for larger scenes where there is need to model plane to hide sky leaking from ground. It works independently to albedo.

sun is not available maybe but in luxrender(spectral) it is

ArHosekSkyModel.h Version: 1.4a, 2013-02feb

Solar Radiance Function

For each position on the solar disc, this function returns the entire radiance
one sees - direct emission, as well as in-scattered light in the area of the
solar disc. The latter is important for low solar elevations - nice images of
the setting sun would not be possible without this. This is also the reason why
this function, just like the regular sky dome model evaluation function, needs
access to the sky dome data structures, as these provide information on
in-scattered radiance.

CAVEAT : in this release, this function is only provided in spectral form!

RGB/XYZ versions to follow at a later date.

luxrender allows optional show only
atmosphere, atmosphere+sun, sun-noSky(innacurrate, lacks attenuation)

but allowing to specify as separate objects would that allow for multi-importance optimizations?

the werid below-ground color

isn't that irrelevant?
i don't think anything below the horizon is supposed to be real data,
from the paper it seems it a model-function(faster,efficient) that is designed to fit their simulated(slow) data

albedo

no it essentially means reflected-color here (same as 3 separate albedo's for RGB), not fully implemented in blender
, and recently in luxr1.6