AC voltage sensing circuit
You may ignore current flowing from the mains to the ground (if it is not earth, at least), but you better think about currents flowing between line and neutral. As shown in the picture (I assumed all resistors are of the same value), when current flows in one direction you will always have one opamp with positive input below ground potential (v1 in this case) and during second half-cycle you will have other opamp input below ground.



One option to make this circuit work is to use dual supply op amp (however interfacing it to MCU might be tricky). If you want to use single supply opamp you may want to consider your V1 and V2 points as AC sources and add some DC bias (typically half of the op amps supply voltage) to them as described here. Remember: after you connect your sensing circuitry to the mains entire your circuit becomes unsafe, you should not touch it with your hands and burn your microcontroller firmware only with optically isolated programming tool.