> My use case for these low-level APIs is to write tests for low-level string/encoding handling in my custom use of the PyPreConfig and PyConfig structs. I wanted to verify that exact byte sequences were turned into specific representations inside of Python strings. This includes ensuring that certain byte sequences retain their appropriate "character" width in internal storage.

CPython contains many checks to ensure that a string always use the most effecient storage, especially in debug mode. The C API should not allow to create a string using an inefficient storage, unless you "abuse" the C API :-D

I'm not sure what do you test.