Re: current measurement
If you use resistor on low side, be carefull how you connect it to the opamp on the PCB. Best way is to put two dedicated traces, side by side from resistor pads to opamp pads and use them for current measurement only. Also put opamp as close to the resistor as possibile.
Another way to measure current is to get ICs intended for this - maxim-ic has some low side or high side current monitors that also have output for power measurement.