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The second rule should read

Linking with python_d -> compile client code with _DEBUG
since Python does not care whether DEBUG is defined or not;
it only cares about _DEBUG (i.e. /MDd, as you point out).

Furthermore, the second dependcy is also thanks to
Microsoft: If you compile your library with /MDd, you must
also compile all client code with /MDd, see MSKB Q140584).
Python currently supports two configurations on Windows:
1. Neither of _DEBUG, Py_DEBUG is defined; link with
pythonxy.dll and msvcrt.dll (i.e. /MD); extensions end with
.pyd or .dll
2. Both _DEBUG, Py_DEBUG are defined, link with
pythonxy_d.dll and msvcrtd.dll, extensions end with _d.pyd
or _d.dll.

It would be possible to create a third configuration:
_DEBUG is defined, but Py_DEBUG isn't. Link with a no-debug
pythonxy.dll, but with msvcrtd.dll. You couldn't use the
existing pythonxy.dll and existing extension modules, since
those already link with msvcrt.dll. Adding such a
configuration would increase the mess, so it won't happen.

I'd advise you to pursue a fourth configuration: Do not use
the debug CRT (i.e. compile with /MD instead of /MDd), even
if you compile your own library for debugging. With that
configuration, you can use the standard Python distribution
with no changes.