Is sys._base_executable correct without a venv? It should be the same as sys.executable in that case.

venv calculates 'home' here: Lib/venv/__init__.py#L117

        executable = sys._base_executable
        dirname, exename = os.path.split(os.path.abspath(executable))
        context.executable = executable # not relevant to this bug
        context.python_dir = dirname    # written as "home = ..."

If the path doesn't exist later on, it's because it didn't exist in the first place. *That* could be the real bug.