The enquiry has revealed that improper washing techniques can allow water to penetrate the alpha sensors leading to a multiple sensor failure. (...) I find it worrying that a simple error by relatively unskilled ground staff can lead to such a situation.
Exactly. Improper static port protection technique during washing can cause false static pressure indication, lot of false warnings, crew disorientation and hitting the ocean. You don't need Airbus for that, Boeing 757 will do. Welcome to aviation - where no task, no matter how trivial it might seem, is to be taken lightly.
A multiple failure will not be correctly interpreted by the automation and in this case apparantly caused a control law change and a full-up elevator trim which was not immediately apparant to the crew.
Almost. You don't need multiple failure to puzzle automation or "automation". Sometimes all it takes is one sensor giving false data, instead of having good manners and ceasing to function altogether. Single blocked pitot of B757 doesn't cause BLOCKED PITOT light to turn on. Rather it will throw RUDDER RATIO and MACH TRIM false alerts at pilots. Those who go troubleshooting nonexistent failures while putting the lying ASI in charge of autothrottle can easily stall the aeroplane. Single radalt of the 738 that measures zero while being in the air, can reduce thrust to idle on approach and that can be disastrous if the crew doesn't pay attention. Single malfunctioning AoA probe can trigger the stall warning and make the crew trained to always respect the stick-shaker try to re-land L1011 immediately after the lift-off and wrecking it in the process.
Subtle failures can often be recognized only by the intelligent entities. Computers, no matter how sophisticated, are still very far from being intelligent.
Control law change was effected after the aerodynamic stall. Only when the aeroplane started tumbling did the computers "recognize" something was terribly wrong and threw in the towel.
Instead of elevator trim, A320 uses trimmable horizontal stabilizer. That THS can easily overpower the elevator is the piece of basic aeronautical knowledege that is sometimes forgotten. It shouldn't be. Pages 80-82 of the report refer.