What's the best way to remove harsh shadows from a face?

IME there is often so much damage there is too little information to correct back to a good match with surrounding skin tones, so I paint in the shadow areas on a new layer, blur it to feather the intersection, and mask to take care of harder edges like around the eyes. Then apply texture to match the original.   

Camera RAW > Effects > Grain can work (but not as well as it did in earlier versions where you adjust grain size).

There are brush sets that have very good skin textures like this one by Castrochew that I have been using for more years than I can remember.

It's not what you asked, but you can also greatly improve the masking of flyaway hair by painting in new hair.  I don't use the above set for that so much, preferring my own presets which are simply single pixel fully hard brush with size jitter set to fade.  The example below is at 100% zoom, and brush spacing was set to 5%  You can see I have feathered the start of the strokes with a layer mask, which helps blend the new hair with the old.

The Castrochew hair brushes are still useful for painting in the body of hair.

BTW  I use Lazy Nezumi Pro for brush smoothing, which is important for hair, but that's not an option for the Mac OS.  Photoshop's native brush smoothing is almost as good nowadays though.