https://ardupilot.org/plane/docs/common-compassless.html state that to enable it, one should disable compass use.
1: does it mean this otherwise useful method, is of no help for bad compass data due to installation error, calibration error, or electromagnetic fields?
2: is it an always-active safeguard ? or only active once ?
3: would it handle or help in this event: Crash during takeoff due to bad yaw reset (/electromagnetic interference) ?
4: which version of ArduPlane was the first to have it ?
5: which version of Copter will have it ? (or is it in master now?)