BA3121 datasheet pin confusion
Your re-labelling of Vm1 and Vm1 is not equivalent to yours so you should not change them.

These are midpoints for DC near the mid point of V+ and gnd and are both AC input from CM gnd noise but DC is created by internal fixed R divider and input by an R to base on pin6 whose emitter controls Vm1 bias.

This clever circuit cancels the influence of chassis ground noise by more than 50 dB.