How to calculate instantaneous active power from sampled values of voltage and current.?
Instantaneous power is simply the instantaneous voltage times instantaneous current.  This is true regardless of the power factor, amount of "reactive power", phase angle, or any other way of expressing this same issue.

To determine phase angle (or power factor, "reactive power", etc) takes more than one reading.  Average real power is the average of instantaneous power.

real power = ave(volt x amps)

The power factor is the real power divided by the "VA" (volt-amps):

power factor = (real power) / (RMS(volts) * RMS(amps))

From that you can calculate the real and imaginary components of the power, and the phase angle if you know the waveshape.