Capacitor Voltage Hold up (During Input Power Interruption)
Suppose, I have a single 100nF input capacitor at my 12V


And


  Suppose, the 12V drops to 0V for some, say 100us, and then comes back
  up to stable 12V. Will that 100nF capacitor be able to provide the
  stable 12V


No, the 12 volt dropping to zero volts will drag the capacitor voltage along for the ride and you get no benefit. You need to put a diode between the raw 12 volt supply and the capacitor to stand any chance of holding up the voltage.

$$I = C\dfrac{dV}{dt}$$

That formula will allow you to plug 50 mA into the "I" part and examine how quickly (or slowly) voltage changes against time (dv/dt).

Bear also in mind that the diode will have a forward volt drop of circa 0.6 volts so your actual supply to the thing that draws 50 mA will be more like 11.4 volts. That may be a show-stopper for you.