What I wish to understand is what is good/bad practice, and why, when it comes to imports. What I want to understand is the agreed upon view by the community on the matter, if there's any one such in some PEP document or similar.
What I see normally is people have a python environment, use conda/pip to install packages and all that's needed to do in the code is to use "import X" (and variants). In my current understanding this is the right way to do things.
Whenever python interacts with C++ at my firm, though, it always ends up with the need to use sys.path and absolute imports (but we have some standardized paths to use as "base" and usually define relative paths based on those).
There are 2 major cases:

Python wrapper for C++ library (pybind/ctype/etc) - in this case the user python code must use sys.path to specify where the C++ library to import is.
A project that establish communications between python and C++ (say C++ server, python clients, TCP connections and flatbuffer serialization between the two) - Here the python code lives together with the C++ code, and if it some python files end up using sys.path to import python modules from the same project but that live in a different directory - Essentially we deploy the python together with the C++ through our C++ deployment procedure.

I am not fully sure if we can do something better for case #1, but case #2 seems quite unnecessary, and basically forced just by the choice to not deploy the python code through a python package manager. Choice ends up forcing us to use sys.path on both the library and user code.
This seems very bad to me as basically this way of doing things doesn't allow us to fully manage our python environments (since we have some libraries that we import even thought they are not technically installed in the environment), and that is probably why I have a negative view of using sys.path for imports. But I need to find if I'm right, and if so I need some official (or almost) documents to support my case if I'm to propose fixes to our procedures.
For your scenario 2, my understanding is you have some C++ and accompanying python in one place, and a separate python project wants to import that python.
Could you structure the imported python as a package and install it to your environment with pip install path/to/package?  If it's a package that you'll continue to edit, you can add the -e flag to pip install so that when the package changes your imports get the latest code.
Python sys.path vs import