Before taking off again (or from any new location), do these checks to make sure the compass is not being negatively affected by some type of external magnetic force. If you see any of those issues, you should take off from a different location.
It would be very hard (likely impossible) to calibrate the compass when it’s being negatively affected by some type of external magnetic force. So, if the compass calibration ever fails, you should move to a different location and calibrate again. Also, be sure that you don’t have any type of magnetic metal objects on your person (like on your wrists) while calibrating the compass. Anything within about 5 feet of the drone could interfere with the compass.
As for calibrating the compass in general, check out the DJI Mavic Compass Calibration Guide. It contains everything you need to know about calibrating the compass on the Mavic.
No. A bad IMU calibration will not cause compass errors like those in your flight log above.
Maybe take a test flight in a wide open location (that’s not near the water) so you have enough space to ensure all is in working order.