Your statement, "It is strongly recommended that you never use the bird in case of unreliable instruments" is not backed by anything.
I do not dispute that this is my opinion only. But you could ask any Airbus pilot, instructor and most of all any Airbus instructor and he would tell you the same, in the sense of:
In case of unreliable instruments, switch off everything and fly pure raw data only. First memory items are: autopilot, flight director and autothrottle off. This means that you DON'T switch on the bird (as you do normally in an Airbus when you switch off your FD, e.g. during a visual approach).
Then you start your damage assessment: Where I am, what I am flying, where am I going, what is working, what are we doing.
When you are stable, you have found some reliable instruments, did your switchings aso, you are allowed to try out the bird. Only then. But not before.
One has to realize that the FPV is an even more "artificial design" and further away from raw data than the flight director. That's why it may be even more corrupted than the later. That's why you should never use it with unreliable speed.
OK465, the "flight director" of the bird is called FPD, not FD. Of course it has it's own "flight director" but please don't confuse people with wrong names. FD is generated by FMG(E)S, FPD by IRS.