It is hard to see it all without all the pictures packed (or I just don't have HDR activated), or that the head is lacking materials. However, I myself have had a few troubles in regards to things being not lit the way I like it. One of the things involves having specularity spots on eyes even in a dimly lit room. The two options I came up with was having "virtual lights" (spot and point) surrounding the face, and having a starfield texture added as emission.
I have played around a bit with your node setup, and what I've come up with is that from the
Mix node that is connected to the
Base Color, add a connection to a
Bright/Contrast node, connect that to an
Ambient Occlusion shader, and then an
Add shader that adds together the
Ambient outputs to the
Bright/Contrast node you can control how bright/vibrant you want your eye, seeing as
Add) doesn't have a slider for how much it affects the material. If you want a "strength slider" for the shader, or don't mind the eye emitting some light, you can exchange the
AO shader for an
Hope this helps.