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When trying to use an "external" Python (with the appropriate PyWin32 installed, obviously) with Softimage 2015 R2-SP2 you may run into unexpected crashes at startup.
The way to solve this is to create an environment variable called "PYTHONHOME" and set it to the path of your Python installation (i.e. something like "C:\Python27".
If you want to revert back to "internal" Python, you'd have to delete the environment variable again.

One more thing: when using an "external" Python, chances are the "print" command will stop working and you'll be forced to use "LogMessage" again...

try replacing your external python with the one from Maya 2015/2016, it's compiled with the right compiler to get 'print' to work. Pyside is also correctly ported to 64-bit in there. I have not understood why you ended up using an external python, however, so this might defeat the purpose.

Thanks. I ended up trying an "external" Python out of sheer desperation, quite frankly... ;)
And the newer incarnations of Python have "pip" integrated, so you don't have to find a way to install this in the "internal" Python.
But that task was relatively simple, so I now have "pip" installed in "internal" Python as well.