The difference is that we get smoother shading from doing the Phong reflection model in the fragment shader because that way we can get around the linear interpolation that shortens direction vectors when interpolated into the fragment shader.
Flat shading does not blend colours between faces while Gouraud shading does a simple blending of colours between neighbouring faces and requires a lot of triangles to look good, Phong shading is smoother than Gouraud and has better highlighting than the other two while also providing more options for lighting properties, therefore, requiring fewer triangles than Gouraud but more calculations per triangle and is therefore in my opinion if not ultra-performance sensitive the best option.
A directional light emits all its light in a specific direction and nowhere else like a flashlight does irl while a point light emits light in all directions like a normal lamp you might have without the lampshade.
Short answer is yes. If the calculations are done in the eye space then it influences the direction toward the observer and the light position else it's just the lights position and direction in relation to the camera.
It removes the reflection of the light in the object. This is a similar reflection to the one the sun makes of its self in water or if you shine a flashlight into water at the right angle.
Increasing the shininess decreases the area of effect that specular has meaning the reflection of the light becomes more focused.
I have computed the light in the eye cordinate space.