Maybe setting "poStderrToOutPut"

BINGO! That was it. Now it can capture the output even as root.

I had a look at wpa_supplicant.conf and found it is an XML file having
to do with dbus apparently.

For the last five years I have been using ifup manually to connect
whenever I didn't have a network manager. It always worked and the
connection is stable even for weak signals.