Analyze op amp circuit
R7 does not do anything of value, it is actually harmful (increases output noise and output offset voltage- and related drift). Ideally there is no voltage across R7 so it has no effect, but op-amps are not ideal and if you think about the resistor approaching zero it's clear there will be problems. You can calculate the effects if you model the op-amp with  Vos and Vn voltage in series withe one of the inputs (offset and noise). 

Vout ~= \$ -V_{IN} \cdot \frac{R_2}{R_1} -V_P \cdot \frac{R_2}{R_4}\$, ignoring op-amp gain and offset voltage. 

You can find that by applying KCL to the non-inverting input node, assuming zero input current to the op-amp and then find Vout such that V- = V+ = 0V.