"When FADEC wants EPR mode to stop being used, Engine is held at present values and crew action is reqd ( press a button to select N1 mode ) and only then will Engine respone to Thrust Lever commands? "
It's not (absolutely) necessary for the crew to select the alternate N1 mode if the EEC's have already done it automatically in response to specific failures (however, if one has gone to N1 mode and the other hasn't, then for reasons stated above, you would manually select both to ALTN).
One of the reports said the pilots lost their EPR's. This is an indication that perhaps both the EEC's had switched modes (automatically).
The Flap Slat Electronics Unit tells the EEC's that the flaps have been extended and the EEC's select approach idle (as opposed to minimum idle). Gear extension produces "intermediate idle".
Yes, many aircraft systems still have old-fashioned relays, all prone to failure over time (as anything is). However, depending on the circumstances, the crew are informed that the engines are not in the appropriate idle state.
Rgds.
NSEU