For your power combiner, this is a special case where the signals coming in to ports 2 and 3 have a special amplitude and phase relationship that allows all of the power to go to the port 1. Therefore the S22 and S33 return loss values are much better and the receiving efficiency is the same as the transmitting efficiency.
On the tapering of the arrays, the usual thing to do is to feed the left/right elements equal distance from the center from the same power divider. Unequal dividers are used to make the taper.
In one sense the tapered array is less efficient than a uniform array because the peak gain is less. But all of the power coming in from the transmitter goes into space. As far as as beam patterns, the tapered array has a lower peak but a broader beam width so the total energy radiated is the same.