All my cameras are set to 16:9 but the mini viewport is stuck showing 4:3 which is the whole work area and not the camera. This makes composing shots for 16:9 impossible. Is there a setting somewhere to change this?
What are your output render settings?
This will determine the aspect ratio of the viewport and the mini viewport.
The aspect ratio of the camera has no influence on that. I just loaded an older project and the camera is at 1.5, but because my render are set to HD, I see the required aspect ratio.