I have a problem understanding which resistors set input impedance of an instrumentation amplifier. What is the input impedance of the circuit given and what is the purpose of R250 (10k to GND)?
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R250 is needed to allow natural bias currents from the op-amp inputs to flow to local 0 volts. If it were not there, the inputs would float and likely cause common-mode input range problems.
The differential input impedance (S2A and S2B in current positions) is R163 + R164 = 1022 Ω. This increases by 28 kΩ when S2A and S2B are changed over.
