Why are all but one sliders not accessible in this dialog?
this is the case when the option Scan type is set to Line Art Scan. This is intended behaviour.
How can I best use this dialog to optimize my preview?
You best use the mid-tone slider first (top slider). This slider ONLY changes the middle tones of the data inside your scan frame. This will not spoil an image as not all tonal values are changed alike.
What is the difference between L and N position of the button for the top slider?
The L position indicates that the mid tone values will change lograrithmically upon moving the slider.
Whereas the N position indicates that the mid tone values will change linear (numerically) upon moving the slider.