Airlines don't publish their international "unofficial" upgrade policies because they usually don't have any written policy. There are obvious reasons for that - the more predictable the process is, the more likely frequent flyers are to "play" the system.

Regarding the others theories:
OutOfOffice - it's true that when OpUps occur, time is limited, which explains why gate agents and concierges are empowered to make such big decisions when airport staff usually are not allowed to have anything to do with revenue management. So while there may be a generally accepted pecking order, gate agents can always avoid criticism using the argument that getting the flight out on time was more important than sticking to unwritten rules.
otralot - revenue management often will not open upgrade seats if coach is oversold because leaving Y at 0 forces people to buy into J / F. why they don't do it when J is >9 is Y is -23, I have no idea. Also, revenue management and all those related systems are complicated enough - riddled with "exceptions" and the like. Asking for more exceptions for various reasons isn't worth the trouble - more confused agents and more confused, angry passengers...

I know for a fact that at least one transatlantic carrier specifically does NOT op-up elites because they have so many problems with elites who begin to expect it when coach is oversold and make a scene.