First : Try to reduce the speed until you have correct setting using Arduremote
The random mode is very strange ,certainly something is wrong with IMU orientation or left right wheel inverted.
What you see is the roll (using odometry) and accel(using odometry) and forward (using IMU) and the IMU heading is not corresponding with the last roll (using odometry) heading, so mower do a big circle to find the IMU heading and start the ZIG ZAG because the IMU PID is not perfect
Into manual mode when you follow the mower if you hit the left button mower need to roll left ,please check this
In WAIT/OFF mode, Into setting IMU : if you roll the mower by hand Counter clock wise : the GYRO YAW need to decrease and increase in Clockwise roll.
In bylane :when mower go forward (into setting Odometry the ticks count need to increase)
Can you show a picture of the location and orientation of IMU inside the mower ?
In BYlane with 0.1 and 0.2 setting did the mower find again the heading in less than 2 seconds if you move it with your foot.It's possible that a so small PID value result in no action at all and mower simply go straight because the 2
motor run at same PWM.
Into mowing mode (real time setting) Try to start with P=4.4 and I=3.3 and reduce the P value gently, but first reduce the mowing speed.