I agree that differential voltage divider is the appropriate solution for most AC voltage measurement applications. There are cases where the measurement circuit ground is tied to one input voltage terminal. Then a single ended voltage divider as used in post #1 does the trick. The amplifier adds an offset voltage to achieve a bipolar range but doesn't involve a substract operation because input and output ground are identical. Respectively there's no CMRR figure.
The circuit in post #5 is referring to a difference amplifier, as probably noticed, the resistor ratio is adjusted for the 3.3k voltage divider output resistance. If I understand right, you don't know how to calculate the circuit gain. But that's rather trivial, two cascaded dividers and a non-inverting amplifier. I think it's more straightforward than referring to a modified difference amplifier calculation.