Engines:
Nice post...
I see 'airworthiness' as a subset of safety, and it would be better for everyone concerned if we let the term 'airworthiness' fade way.
I respectfully disagree. Airworthiness cannot "fade away" because it is the start of safety. Without it safety is impossible to judge. In theory, a signed RTS indicates that, accepting as yet unknowns, the airframe being inflicted upon the crews is safe and fit to fly. That the various systems function as advertized and, as far as can be reasonably tested, they don't interfere with the proper function of the other installed systems. It also lays down limits on how it should be flown and those limits allow for some "flex" to build in a safety buffer. There's the baseline of your "safety" right there.
From that point on every change, addition and subtraction, (as you noted yourself), should be tested and
documented. There will be things that manifest themselves that weren't tested or couldn't be properly tested. These need to be properly fixed or mitigated, noted and included in the baseline. As you've seen yourself the lack of proper documentation causes all kinds of problems for the maintainers... It causes death in the operators.
Safety comes from the baseline and the baseline is the RTS that says this airframe, in it's current state is "fit" to fly with crews in it. Airworthiness is the mod state that is the sum of the fully documented changes to the RTS.