1-a) Because of the lifetime of the intermediate state. Highest probability is for the decay to happen almost instantaneously, and the probability exponentially decreases as time increases.
1-b) This is because of the probable photon emission direction shown in the slides before. The highest probability overlaps with the nuclear spin orientation, and the lowest one- with the direction perpendicular to I. This is exactly the difference between detector combination 1-3 and 1-2