How to measure the input impedance of a summing amplifier?
Anything wrong with the age-old trick for measuring the input impedance of a "black box"?  Create a voltage divider of a resistor and the input to be measured, selecting a type of resistor or variable resistor that will behave as a pure resistance at the frequency involved.  One then substitutes resistors, or adjusts the variable resistor, until the signal amplitudes across the resistor and the input are the same.  The DC resistance of the resistor should then be the same as the input's impedance at that frequency, no?  (If a variable resistor is used, it would be removed from the circuit and measured with an ohmmeter.)  Of course, this is a simplification.  A simple calculation would avail with a ratio of amplitudes other than 1:1.