I want to be able to paint the eyebrows of the mesh without paint overshooting onto it's forehead. How can I either:
Adjust my workflow to stop such things happening or
Adjust my brushes' settings to stop the overshooting
In the Texture Paint mode you can enable a Paint Mask (red arrow, screenshot 2). When it's enabled then you can only paint on faces that are selected in Edit mode.
For example, switch to Edit mode and select the eyebrows, eyes, nose, and inner mouth. Then switch to Texture Paint mode and enable the Paint Mask.
Now you can paint the eyebrows, eyes, nose, and mouth without overpainting the color of the face/head.