Question 5: Why including more digits in answer, does not make it more accurate?
Accuracy of a measurement is its closeness to the actual value. If the difference between the measured value by the experimenter and the actual value of the quantity is less, the measurement is more accurate. It does not depend on the number of digits in the measurement. For example, if the actual mass of a body is 100 kg, then 99.8 kg is obviously more accurate than 98.2531 kg.