"Personally, I am 100% against letting an embedded runtime automatically pick up any settings from the environment. You don't know where they came from."

Why is that?  When the embedding application is using Py_SetPythonHome there is clear intent to use a specific python setup.  This is no different to using any other Python setup.  Why venvs should be treated differently that standard installations?

As to manually setting the path, why pass the burden to the embedding application.  The idea is that python, in the embedding application, should work as in the python executable.   Trying to match the complex and evolving algorithms that python uses to setup the path, is a huge and unnecessary burden on the embedding application.