Dynamic import script module, importlib can be used?

Not sure. I have my doubts.

You certainly should never import any Ignition project library script. Never. (Ignition uses its own "import" machinery.)

Sure, as long as the ultimate origin of the call stack is in project Final.

Such a function in Global would depend on the ultimate caller's context to have an implementation of the named function. Different leaf projects could have different implementations.

Edit: This long-standing script library bug in Ignition's "auto import" behavior is relevant here: