The thing you need to know is that the right diagram, with the two dielectrics directly in contact, is the same as the left diagram with two separate capacitors.
So if no current is flowing the voltages across the two resistors must be zero because for a resistor V=IR.
R1, R2 and C2 are in series, so the total voltage across them has to 90V. Since the voltage across the two resistors is zero that means the voltage across C2 must be 90V.
(I've just realised I mislabelled my diagram. The lower capacitor is C2.)
Yes. And C1 and C2 are in parallel with R2 and R3, so the voltage across C1 must be the same as the voltage across R2, and the voltage across C2 must be the same as the voltage across R3.
@Nobodyrecognizeable yes, that's correct for the switch open. So you've answered the question haven't you? Now you know the voltages the charge is just $Q = CV$.
Ah, yes, well if the voltage at the bottom end of C3 is zero, and the voltage drop across C2 is 75V, then the voltage at the other end of C2 must be 75V. Yes?
Or look at it this way. The voltage at the top end of C2 is +100V, and the voltage drops by 25V across C2 so the voltage at the bottom end of C2 is 100 - 25 = +75V.
Both approaches give the same voltage of +75V at the point A.
