No. Adjustments you make to a photograph in Lightroom do not change image pixels. Adjustments are recorded as instructions in the Lightroom catalog.
Yes. The History panel keeps track of all the adjustments you make to a photograph forever, unless you delete history states.
No. You can make some local adjustments in Lightroom, using tools like the Graduated Filter tool, the Radial Filter tool, and the Adjustment Brush tool. If you need to make a more complex local correction, such as one that requires a precise selection, then pass the file off to Photoshop, usually after making fundamental global corrections in Lightroom.
Increasing or decreasing the Exposure slider affects the overall brightness of a photograph. The Exposure slider has its primary effect on the midtones in a photograph.
No. White balancing is subjective. Lightroom’s White Balance controls can neutralize a color cast in a photograph, but there are times when a color cast is desirable as a way to communicate the mood and message that you, as the photographer, choose to convey.