Why make such a simple transit so bloody difficult???? Just set the QFE for the few minutes you need it then back to QNH.
Because if you’re distracted during high workload conditions you may forget to reset QNH. Or you may mis-set it.
Most countries do not use QFE.
In the not too distant future altimetry will be GPS based and altimeter setting (and mis-setting) will be a thing of the past.