Given an a.c. adapter, it is real power in watts. Without, it is apparent power in VA using your assumed value of voltage.
It can be, but you may well run into problems with the amount of data that is stored on your SD card. We use 10 s as the normal sample rate, and the normal sampling period is 200 ms per input, so you can’t go much below 1 s. You will need to alter the sketch in the Atmel 328P (the emonTx part of the emonPi) to change the sample period and rate.
How accurately do you know your voltage, how steady is it, and is apparent power (VA) good enough? If the answer to the first or second is “not very”, or the third is “no”, then you need the a.c. adapter. If you need to know the direction of power flow, you need the a.c. adapter.
The maths does the correct calculation. For real power, each pair of voltage and current samples are multiplied together and the average power over the sampling period is calculated. If the voltage is assumed, the rms current is calculated by squaring each current sample, averaging and then the root is extracted, finally that current is multiplied by the assumed rms voltage.
I don’t know MyElectric, so I can’t answer that. But if you wanted, you could modify the MyElectric software yourself.