How to determine dielectric constant and loss tanget of a PCB board at a specific frequency

The displacement current only refers to the current flowing in the wires of the capacitor when it is being charged or discharged. Since the charges cannot jump from one plate to the other, the charges are instead displaced, building up on one plate and depleting the other, hence the term 'displacement current'. This charge displacement results in an increasing electric field in the direction of the current flow. The increasing electric field generates a circulating magnetic field that would have the same magnitude as if there was actually a current carrying wire there. Since electric and magnetic fields can be present in free space, this effect can be seen in a vacuum.

This turns out to not be helpful in the way you are suggesting. The capacitance of the combined structure will be limited by the lower dielectric constant materials.

The way you model this type of structure is to just treat it as three capacitors in series. You can mentally imagine that there is a thin conductive plate at each interface. So the overall capacitance adds using the normal rule for series capacitors: Ctot = 1/(1/C1 + 1/C2 + 1/C3)

Each of the three capacitors will have the same charge 'Q' as the entire capacitor. So you can figure out the maximum charge for each capacitor with the equation Qmax = Cmax*Vmax. Whichever is less will set the overall charge (and therefore voltage) limit for the compound capacitor.