I need some information about:
1- you can't with python. (I'm not even sure you can with c++)
2 - I'm not sure to fully following you. I don't understand the "render preview image" is that a command? In the preferences there is a "Render scene" option. That's maybe the version I'm using. (R24)
To fix the texture path in your material presets, you would need to set the preview with a file save on your harddrive. But this is not possible with Python.