The original question "what happens what FADECs fail?" is ill-posed.
They can fail because the units fall off the airplane. In that case, there is unlikely to be anything resembling engine control.
I am guessing that TO MEMO really meant something like the following: what can you expect in the way of functioning engine control if you see such-and-such an ECAM message?
Could one maybe rephrase the question in this form? That way, it is likely to have a definitive answer. If not, then not.
PBL