John,
Perhaps I can help here. Your assumption on F-35 FBW fail modes is spot on.
Regarding auto eject, I remember that it was driven by the possibility of a catastrophic lift fan failure leading to very rapid nose down pitch to an attitude outside the seat envelope. I think that it's triggered by high pitch rates when the aircraft is in the powered lift mode, below certain speeds.
Best regards as ever to all those trying to keep the aircrew safe,
Engines