I never mentioned AC13:) Neither is it about custom tools or locations.
To solve this issue in AC12, you have to drop the corner window tool in favor of the simple window tool. In all simple windows you can set the custom corner option. In that, you can manually define the connection on both ends of the window. Consequently, you can set the corner connection to be on the left side of the window. (Note: corner window tool always puts the corner on the right side of the window.)
In AC14, you can simply change the opening direction of the window - well, in 2 months from now...
Cheers,
Zsolt Táskai
ArchiCAD Development - GDL Team
AC13, AC14 and upwards...