Neither left and right nor up and down are inverted by the mirror-but front and back are, as the author's sister Marjorie illustrates in Figure 27.8. Two surfaces of the mirror reflect light. When the source of glare is somewhat above the horizon, a vertical window will reflect it to people in front of the window. By tipping the window inward at the bottom, glare is reflected downward rather than into the eyes of passers by. A window both transmits and reflects light.
Percentage is one thing, total amount is another.
The pebbly uneven surface is easier to see. Light reflected back from your headlights is what lets you see the road. The mirror-smooth surface might reflect more light, but it would reflect it forward, not backward, so it wouldn't help you see. Whereas diffuse reflection from a rough road allows a motorist to see the road illuminated by headlights on a dry night, on a rainy night the road is covered with water and acts like a plane mirror. The minimum length of a vertical mirror must be half your height in order for you to see a full-length view of yourself.