I think you can leave COMPASS_USE as 1 if you want to use it as a fallback. The EK3_SRCx_YAW parameter values are the ones that control what sensor (compass vs camera) are used by the EKF for estimating the yaw.
For the fallback to work though you’ll want to be sure to align the camera and compass (using the “Viso Align” auxiliary switch) before takeoff otherwise the fallback could be a bit rough as the EKF will suddenly change direction and this could lead to the attitude and position controllers moving the vehicle around a bit especially if the vehicle was moving at the time of transition. I think when the EKF heading changes we don’t automatically rotate the “desired velocity”… anyway, maybe this is too much detail at this point.