To answer you first: the electrometer is not connected in parallel with the switch ~ I don't quite understand why you say so. Note that the electrometer and switch are not connected together at the power supply.
The essence of this circuit is that the power supply charges the capacitor AB through the switch S; At the same time, the electrometer measures the potential difference of the capacitor AB (that resistance is a decoration, because the electrometer is equivalent to an open circuit, and there is no current anyway), so as long as the switch is closed, the potential difference stored in the capacitor AB will not change. Only when the switch is off (at this time, the charge of the two-stage capacitor AB is fixed but the voltage is not fixed), according to the capacitance principle, because the distance between the two poles increases, the potential difference increases, so a is selected.