On the current shunt, the long wire connection is totally unsuitable for accuracy. The shunt is generating useful information in the micro-volt range which will never make it to the A/D converter in the main box without being destroyed by noise. The far better option is to put the differential amplifier on the shunt and return a much larger voltage or a differential signal back to the main controller. This would drastically improve the current monitor accuracy instead of seeing it bounce around by 10's of amps. Yet another step up is to put the A/D converter on the shunt as well and return the data in a digital stream like SPI, I2C, CAN or whatever. It is the same components, just put in different places.