What you are doing is way too complicated for what is actually required.

And the condition that seems to best maximize this is, "copy elision of a named variable is guaranteed if its lifetime does not overlap in any scope with another object that could also possibly be returned".

It is a simple sentence to change in the core language, it allows you to do everything you want in all circumstances that are possible, you don't need to write code differently, it doesn't do anything that the compiler isn't already allowed to do (but decides not to), you don't need to add a new library features or functions or keywords or hoops to jump, no gotchas, you can't do it wrong, you are able to express intent in a coherent manner and allow the compiler to do its job,
It always works and everything benefits regardless of it being immovable/uncopiable or otherwise. That's it, you don't need anything else.