[CARDBOARD] switch on and off stereoscopic rendering

I’m using the official Google VR plugin for cardboard. In our app we need to have a 2d menu (from which the 3d scene can be launched). For this I can disable the stereoscopic rendering, but even if I do this, a vertical line stays on the screen with settings gear. How can I disable/remove it?
Also when the app starts up, right after the splash screen a default image is shown with the text: place your phone into your default cardboard viewer. How can I change this image?

Thank you for your responses.