How can I explain a net flow of electrons in this circuit?
The crucial fact to keep in mind is that each plate of the capacitor has a vast amount of mobile electrons.
To charge a capacitor, mobile electrons are 'pumped', via the external circuit, from one plate of the capacitor to the other plate in order to charge the capacitor.
This leaves the plate that supplies the mobile electrons positively charged and the plate that receives the electrons negatively charged.
A capacitor discharges when the external circuit allows electrons from the negatively charged plate to travel 'back' to the positively charged plate.
To the external circuit, the capacitor is not an open circuit even though there is no conduction current through the dielectric.
From a circuit theoretical point of view, there is an electric current through the capacitor and that current is proportional the time rate of change of the voltage across.
From an electromagnetic point of view, there is a displacement current (changing electric flux) through the dielectric and this current produces a magnetic field in the same way that a conduction current does. So, on this view, there is a current through the dielectric.