Current in a RC circuit

Capacitive reactance is a description of behaviour of capacitor in AC circuits, where current oscillates harmonically and all transient phenomena in the circuit died away long time ago.

In case we study discharge of the capacitor through a resistor, there is no AC current, just a transient discharge. The discharge requires more detailed description in terms of differential equation in time, the concept of impedance or reactance is not applicable.

No.both are considered to find the current let the alternating voltage varies in sinusoidal form the current is given by the voltage equation (sinusoidal form with phase change as there is phase difference between the impedance vector and the resistance) divided by impedance(which involves both resistance and capacitive reactance.