"Bruce Palmer"

How many other situations exist that nobody knows about?

A few hundred.

Why can't anyone pinpoint the reason no
inspection was previously done? Was it a requirement that was
overlooked?

The thing was assembled wrong. It was not obvious to casual observation.
One day a guy was looking at the tech manual and the assembly and said "Hey,
wait a minute....."

I propose that the first manned
flights include one crewmember from the design and engineering teams.

If this scheme were implemented, you'd still have a few parts put in
backwards.