summing amp - floating inputs
Floating input of an op amp is usually not a good idea.


Yes, but you are not causing the "real" input to float...

Open circuiting the 47 kohm resistor that feeds the virtual earth summing input is not going to be a problem. The 47 kohm feedback resistor shown in your diagram will keep the op-amp biased at the right point.

Shorting to ground the signal that feeds the 47 kohm input resistor may not be such a good idea and you can probably get away with a switch that just disconnects the signal from the 47 kohm.