Current relationships across a voltage step down convertor
Forget about the SW1 switch open or closed it doesn't matters. Because the input and output are not completely isolated(means -ve will be common for input and output).They are already interconnected inside the buck converter itself. The answer to your first question is that in theory ,you can take 96mA output from A2 but it still purely depends on the load.  Buck converter efficiency will vary depending on the load. If you have lighter load on the output side,the efficiency will be much good. If you have more heavy load,then it's efficiency will start to degrade.The degraded performance is due to dissipation of heat due to heavy loads. So practically it not the case that you'll always have constant efficiency irrespective of the load at the output of the buck converter.