All excellent points, but, ultimately, pointless.
(b) when it comes to STCs, a basic and underpinning consideration is that the STC is applicable ONLY to the configurations implicit in the STC S/N applicability range. That is to say, the STC folk are saying that their mod is OK to install on a bog standard aircraft (as defined by the applicability so it may not be OEM bog standard) - but if it has had any OTHER mods then all bets are off.
Can you explain to me 'not quite' and still align it with your previous quote above?
I am yet to have an EO done on my aircraft that the person issuing and signing the EO is even to clap eyes on my aircraft.
He gets some paperwork from the guy fitting it, rubber stamps it, sends it back with a ridiculous bill and an AFM supplement that has been put together with the cut/paste function from the documentation put out by the manufacturer of the whatever component is being installed.
And, speaking with other owners, they report very similar, if not identical stories.
Complying with a lot of these regulations has very little to do with actual airborne safety. I am sure that there are plenty that do, but more than a fair share are just pointless paper chasing exercises designed to liberate money from owners pockets.