Nearly every rejected document photo has the same problem: the person taking it cast a shadow across the page.
Adjustments work on the whole image at once. Brightening enough to lift a dark corner also washes out the part that was correctly exposed, so a heavy shadow ends up trading one problem for another.
The fix at the source takes ten seconds. Turn so the light comes over your shoulder rather than from behind you, or move the page nearer a window. Daylight from the side produces an even page with no shadow to remove.
If the document is already gone and the photo is all you have, black and white after a contrast lift is the most forgiving route. It will not be perfect, but it will usually be legible, which is what matters.
Raising brightness and contrast lightens the paper, and the Clean scan preset does both at once. A soft shadow becomes much less obvious. A hard shadow with a sharp edge, such as the outline of your phone, will still show.
Taking the picture to black and white removes any colour cast from the shadow and helps the text stand out. Nothing here can truly rebuild what the shadow hid, so the best fix is a new photograph with the light coming from the side or above.
Rarely. Adjustments affect the whole image, so a strong shadow leaves a trace. Retaking is far more effective.
Indirect daylight from the side, with your body out of the light path. Overhead room lighting is what causes most shadows.
No.
A little. It removes the colour cast, but the darkness of the shadow itself needs the brightness and contrast settings, or a new photo.