Q2: The extrema are always at the points that the denominator approaches zero. Have a look at equation 2.14; the denominator will be 0 every half cycle and will becoming zero from a positive value for every first half-cycle and becoming zero from a negative value for the second half of the cycle. This repeats for every cycle.
The numerator sin function is in phase with the denominator sin function and every time the denominator becomes 0, the numerator does as well. If N is odd (e.g. 1), both the numerator and denominator become zero from a positive value together, or from a negative value together. Positive divided by positive is positive and negative divided by negative is positive; hence the always positive extrema for N is odd.
When N is even (e.g. 2), when both num. and denum. approach zero, they approach from different directions (i.e. polarities) every second cycle and consequently the result is -1, +1, -1, +1, etc.