You could either re-download the file or right click each slider individually and you'll find *Reset to default value".

Everything can be customized through the N menu (Make sure you have the Eye object selected. (check your outliner) Customization includes: Changeable Iris' with infinite color options, color-able sclera, pupil dilation as well as control over the shape, iris size, vein irritation and each iris has their own normal and displacement which can be combined with others to get differing results.

If you want your eyes to have identical settings that update together, you can duplicate the whole rig with ALT+D, this will create a linked copy. You can also Shift+D but this will not edit in unison.

If you have experience with OSL you should be able to understand most of the nodes, I thoroughly explained a lot of the logic within the Node Groups, the shader is designed to be easily altered in case I wanted to update the product in the future with new textures.

Yes. But, this product is designed for blender, however the textures and model are there for you to set up yourself, unfortunately OSL can not be directly imported into game engines. A lot of engines are compatible with shape keys so the pupil and iris control should carry over.

This is an issue relating to missing textures. You most likely haven't extracted the files from the zip, if you still have missing textures after it has been extracted you can try "Find Missing FIles"