Min. Shading Rate could be lower, anything between 0.2 - 0.5, but it could increase your render time. One is perfectly find for now.  Stick with the manual render settings, it gives you a little more control than automatic.

About IBL, in most instances, IBL requires a light probe or HDR image connected it.  It draws lighting data from the image to create an ambient light environment to fill in the shadows and brighten up the scene.  It is an easy way to simulate natural, reflected illumination from surrounding surfaces.  Commonly, the same light probe or HDRI is also used as a background image because it's lighting data matches exactly from the IBL's output.  If we are strictly interested in only observing how the character's skin and clothing textures respond to light, the IBL would be created without any image attached, and allowed to ambiently illuminate the scene with just a white color.  In your case, 15% might be too bright because it is broadcasting white, but it is a good starting point.  And if it needs to be changed, we could decrease the intensity to suite the scene.