> PYTHONHOME should point to the Python installation root, period.

And indeed, it does. After your proposed, changed, it wouldn't anymore.
The Python installation root is *not* the directory where the Python
library lives, but the parent directory in which both to find the python
executable (bin/python), and the library (lib/pythonX.Y).

As R. David Murray explains, there is no chance that this definition
could possibly change - it would break existing systems.

I'm not quite sure what you want to achieve: if you want an additional
directory to show up in sys.path, just edit Modules/Setup.