When exactly does EKF revert to DCM?

ArduCopter Copter 3.3
1

We’re trying to debug some funny attitude behavior while flying in Attitude Mode. With EKF_FALLBACK set to 1, at what point does it switch from using the EKF filter to the DCM:

a) As soon as the error in any sensor (ie. GPS) goes over the gate threshold (ie. when EKF4 - SP or SV > 1)

OR

b) Not until a error-2 message is thrown (causing the pixhawk light to blink red/yellow), which happens after several seconds of SP, SV > 1

Also, is there any way to view the DCM attitude estimate in the dataflash?

Lastly, are there any serious dangers with setting EKF_FALLBACK to 0, as long as the gate parameters are set reasonably on the GPS and mag?

Thanks!