When Vin is low the enhancement type NMOS is off. That means the drain current of both transistor is zero, isn't it?
If it is, then how will the parasitic capacitor charge? I'm not understanding! And if it charges, how much will the maximum capacitor voltage be?
The depletion mode device is ON when its Vgs = 0, as in your case. So it acts like a small resistor through which the capacitor can charge, even if M1 is off.
A depletion mode N-mosfet has a channel already formed in the silicon and doesn't need additional gate drive to turn it on. In fact you must apply a negative Vgs to turn it off (its threshold voltage is negative)!