Indoor and evening photos come out dark, and the obvious fix, winding brightness up, usually makes them look washed out instead.
Brightness lifts everything equally, including shadows that are meant to be dark. Push it too far and the picture turns flat and grey rather than bright.
Contrast pushes dark and light apart, which is what makes a picture look properly exposed. A small brightness lift with a larger contrast increase almost always beats brightness alone.
If the photo was taken under yellow indoor light, pulling saturation down slightly often looks better than trying to fix the colour directly.
No. What you see is a preview. The file is only written when you press the button.
No. These are straightforward adjustments, predictable and the same every time.
No.