If I were having the problem and really wanted to narrow down the culprit, I would eliminate the volumetrics and test and then eliminate the emitters and test. This changes the atmosphere and lighting but might reveal the culprit, if it's not the shader. This would just be curiosity.

I'll try and track down a link to an older version of EZSkin, but it might take a while. Prior commitments. Maybe someone else has a link handy?

At this point I don't think it's the shader. Seems like a lighting thing. And the irradiance cache route doesn't seem like a path you want to pursue...