@rmackay9
Will give rc1 a try tomorrow.
Looks like the “PreArm: EKF attitude is bad” issue, that I described here and here is still open. This means, a copter configured for GPS-for-Yaw still can not be armed (e.g. for motor test) inside a building without GPS as compass is only a fallback after GPS-for-yaw has initialized yaw at least once.