confused about diode polarity vs circuit polarity
In 4.2b, you have +10V at the top of the circuit, so the cathode must be more positive than the anode.  In that case, the diode acts as an open circuit - no current will flow, so there is no voltage drop across the resistor, and 10 volts, with polarity as shown, across the diode.

The + and - signs beside the diode indicate the polarity of the voltage applied to the diode.