Hello again,
One image speak as a thousand words :)

In the first shot I do a EntityVisible between the ground and the light position, you can see that it produces this artifacts on the ground.
So we got two options, or move the ground away while checking visibility (so the light won't pick it), or we make all the objects double sided, as can be seen in the second screenshot.
I've tested it with complex levels and doesn't seem to be producing any visible artifact, check these screens:



I understood it correctly, the problem you mentioned of a lumel in a corner wall, doesn't exists. Because the lightmapper will split the wall into two distinct surfaces.
One image speak as a thousand words :)

In the first shot I do a EntityVisible between the ground and the light position, you can see that it produces this artifacts on the ground.
So we got two options, or move the ground away while checking visibility (so the light won't pick it), or we make all the objects double sided, as can be seen in the second screenshot.
I've tested it with complex levels and doesn't seem to be producing any visible artifact, check these screens:



I understood it correctly, the problem you mentioned of a lumel in a corner wall, doesn't exists. Because the lightmapper will split the wall into two distinct surfaces.