Clandestino,
System design very well explained except for the case where two similar but erroneous inputs are sensed by the ADRs (from Pitot/Static system). In such a case, the aircraft doesn't know that it's being fed inaccurate speed information and will try to follow the erroneous signals believing that the voting process is disregarding the "outlier" (quaint Airbus terminology) signal which happens to be the only valid one.
In the sim I have seen pilots become totally confused in level flight at a safe altitude when faced with apparently conflicting information on speed. To be fair to Airbus, they do have that "big red button" - Disconnect the automation and fly the memory pitch/power values. The real problem is in being able to recognise the fact that you have a problem. How I'd react if all that happened as I was turning base after an uneventful(?) 90 minute flight, I'd rather not speculate.
In the Excel case, it seems apparent that whatever happened, it was so unexpected as to take the crew completely by surprise.
Is there any information on the A320 experience levels of the two Excel pilots and were they trained for test flying something as complex as the 320?