Just adding a bit more to this to hopefully make it easier to follow as its getting a bit hard to navigate (for me
).
Concentrate on one input at a time. With the correct opamps fitted then zero volts DC at any point in the selected signal path must be achieved.
The input impedances of any floating inputs are quite high and could be picking up noise which might register on a DVM as the pickup would be asymmetric. To avoid that it would be worth shorting each input that is under test at the input socket itself. A Din shorting plug would be ideal.