Hi Alex,
Re 1) and 2), I can’t answer definitively, but pre iGRID, if Mode-S 3D was disabled, PAW would only send bearingless Mode S data to the EFB (assuming it was being received by your PAW). So if data was NOT being received by your PAW directly, you would get NO WARNING.
I would presume the same would apply to iGRID - as the only difference (as I understand it) is the way the position data is sent to PAW - i.e. via ATOM or GSM.
That said why anyone would want to deliberately disable Mode-S 3D in favour of bearingless warnings only, I can’t imagine.
Re 3), SkyDemon stopped displaying ambiguity circles for MLAT aircraft a long time ago as it was causing issues with another EC device. They just display the aircraft where the EC system says it is irrespective of which EC device is in use (not naming names as I don’t want to start yet another war!).
Regards
Peter