more speculation
All those warnings begin
The crew guesses by turning ADR1 off. Only PRIM2 detects the move.
The ISIS, which is fed by ADR1 input, complains.
But the culprit was IR2 which generated wrong ADR2 output.
Then... (fill in the blank)
edit: the crew guesses by turning ADR1 and IR1 off... So that there is no more comparison to generate a NAV IR2 FAULT ECAM warning, as only IR3 remains "healthy"..