1. In the audioapp sample, AudioInit creates a MessagePort and stores it in o.port. It calls roAudioPlayer.SetMessagePort using that port, and then later Show_Audio_Screen calls create_springboard which calls roSpringboardScreen.SetMessagePort using the same port. Thus messages for both the roAudioPlayer and the roSpringboardScreen will be sent to the same port.
2. The deviantart example starts with a poster screen. It's created in uitkPreShowPosterMenu. When the roSlideShow starts, it effectively acts as a screen, receiving button presses, etc.
--Mark