You're onto something with the last loose sketches you posted.
With renderings, my advice is to try thinking about heads and other objects as geometric shapes. A simple trick to it is simplify them. It isn't a head at all, it's a sphere! How do you paint a sphere? Same principle, the general shape of the head obeys the same rules as a sphere would. You have your areas that face the light source and are brightest (top of head, tip of nose, points of ears), form shadows that show from and are soft and transitional (cheeks, forehead), and cast shadow that block light from an area altogether (brow shadows, nose shadow), which in conjunction describe the forms. We as humans develop an innate understanding of visual information presented to us, so all those things read as specific elements so you can't just use those things interchangeably, because it won't look right. Hope this helps.