Is it totally impossible to use an M3 for mapping, or does the electronic shutter just limit it and can be used within certain parameters?Correct - I don't remember where I read that it did. There's an interesting comparison of the readout speeds of the sensors on the M2P and the M3. The faster the readout speed, the smaller the rolling shutter effect and the more accurate the mapping missions.
M2P readout speed is about 51 milliseconds.
M3 readout speed is about 16 milliseconds.