This "FLCH trap" is not something that can ever happen in final approach if the pilot's have brain in their heads.
Again, this is never something you just assume in life-critical system designs. In fact, you assume the opposite and then look for any possible valid use of said configuration in light of its catastrophic nature if ever fallen into. If there is NO possible redeeming feature of said configuration, then you design its prohibition into the system. "Pilots want the freedom to 'crash into brick walls'" is not a redeeming feature. It's just Darwinian.