This is not Nimrod or Patriot; there were opportunities for the Reds' management, the lineys and the pilot to identify weaknesses or faults and break the accident chain.
It totally goes back to what you expect to see, the photos clearly shows the handle out of it's housing, but looking down on the seat you will be seeing it from a different perspective and to be honest your mind set would be concentrating at looking at the pin being in, especially as you would be unaware that there was a design fault and it would as far as you knew be impossible to fit the pin without everything being correct, hence your attention of focus would be the pin.
The CAA at airshows actually present a light aircraft with built in faults present, split pins in controls missing, screwdrivers in engine compartment, nuts missing off wheels etc then invite members of the public / engineers / private pilots/ commercial pilots etc to spot the faults...... Very very few actually find them all, indeed the majority do not even come close, and they know there are faults present.