What is meant by "adjustment of the output gain and offset" of a Force Sensitive Resistor interface circuitry?
You are correct about #1.  The buffer prevents the load from changing the voltage determined by the FSR/RM voltage divider.

All opamps have offset errors.  This happens because it is impossible to exactly match devices in the differential stages of the amplifier.  Typical values are a few millivolts.  You can model this as a voltage source in either input to the opamp.  R5 and R6 are subtracting this error to make it zero.  Because the value to be corrected is small R5 reduces the sensitivity of the adjustment.  

The single value of R6 is the across the ends of the potentiometer.  The wiper, the part that moves, can divide this with respect to either end from 0 Ohms to the full value of R6.

R5 is used twice because both resistors typically have the same value.

Ultimately in a circuit like this, the gain you are interested in is Vout/Rfsr.  R4 will let you change this gain.  This is just a way to calibrate the circuit.