1-a) Why do you observe an exponential decay ?
This is always the case for radioactive decay, this comes from the differential equation for the decay. Which depends on the amount of excited states that are left.
1-b) Why is the number of events/counts for detector combination 1-2 smaller than for 1-3 ?
As we saw in LTNO decay mostly happens in the direction of the spin or the opposite direction. So this would a higher count rate in this case aswell.