Analyzing RC-circuit with diode
Well the lowest voltage on the output cannot be less than 11.3 volts (one diode drop from 12V) and given that your 19 volt p-p square wave attaches itself to the output when the lowest point hits 11.3 volts, the final output has to range from 11.3 volts to 19 volts higher at 30.3 volts.

Virtually same answer as you - just a difference in what I assumed the forward volt drop of the diode to be. Given that the square wave is 50 MHz the 100k pull down resistor might reduce the peak output by around a milli volt or so.

Also no problem with the capacitor at 50MHz - it'll sail straight through. Given that the diode is really fast you won't get much of an issue with the diode's reverse recovery time response.