A couple of things:
737 rudder problem is not a mystery. They know what caused the accident. Secondary slide jammed in the PCU. There is a new rudder PCU that fixes the problem. They also have a mod that reduces the A system pressure most of the time with the old PCU (lowers rudder travel and therefore crossover speed). Also being shown a rudder hardover in the sim and the procedures for recovery, makes it much easier to deal with. Aluminum structure is easier to inspect than composite.
I have over 5000 hours in the 737 and am satisfied with the solutions to the rudder PCU problems. I am currently an A 320 driver and think that both Airbus and Boeing make good airplanes.
As far as the missing rudder, looks more to me like the upper hinge came apart or something of that nature, not a plastic airplane part failure initially.