have you checked all the wiring? by that motors spin in the right direction? a rough flip indicates that you have a motor/s not spinning in the right direction or propellors wrong also check the oreintation of the FC check the status of the IMU
How did you fix the motor don't arm problem and later did you find a solution/reason for the flip over?
I also can't get the motors of my s800 to arm with the N3 but I also think the ESC calibration doesn't really work:
https://forum.dji.com/thread-204832-1-1.html