I would invert the mask you had, shrink it a bit, smooth it, then inflate in the deformations tab and smooth it a little all over with the mask still on.
Sculpting anatomy is best represented with sharp angles, or else it quickly turns into a smooth ball. Once the features are locked in, it's time to smooth and blend together.
Those are just mesh normal smoothing artefacts. Inside your edit poly find the smoothing group section and experiment with the buttons /read the documentation
You don't have enough geo in the high poly, simple as that. :) SubD modelling is a big topic, but you control the smoothing with support loops and creases to get the smoothing where you want it to be.
Do you mean to ask how I made the normal map like that with no smoothing errors? It's as simple as baking it with 2 separate smoothing groups for those faces.