The sampling rates, measured with a standard emonTx V3.4 (and the emonPi is the same processor) are:
calcIrms( ) - approx 5588 current samples per second.
calcVI( ) - approx 2535 pairs of voltage & current samples per second.
Sampling rate of emonLib
(A search should have found that.)
A calibration coefficient is just that - it adjusts the calibration according to the physical properties of your hardware. That and the others really are explained in Learn. The information is not best organised, but it’s there.