One more thing... You're including your cubemap reflections in your customlighting input, so you don't get reflections (and apparently metallic surfaces are only "lit" by the cubemaps?) on surfaces that aren't facing a light source. It doesn't really make sense that a surface would have to be lit directly to have indirect…
Yes, it would, if the cubemap that was used incorrectly represented an environment that was lit. The cubemaps would actually have to somewhat accurately represent the amount of light coming from the environment for it to work properly.
But if you apply the cubemap reflections to your customlighting/customlightingdiffuse, the reflections that are in the shadow of the main light source will be much darker than the faces that are facing the light source. The environment could bounce more light from the "shadowed side" and therefore result in incorrect…