I have good news for you:
The jaw angle can be changed via a Side Wing. A Chin Wing will make the angle more rounded because the chin has to move forward which is the reason why you should have a Side Wing and sliding genio separately. This means by doing the movements in two parts, you can just drop the gonial angle down, therefore making it look square as it connects back to the mandible then move the chin forward by itself without effecting the gonial angle. Using this method and doing some bone shaving in addition at the angle will make it look even more square.
What are your thoughts?