Q1: Not very elegant, but you mute the radio (or media) by turning the volume down to zero. Has the same effect as a proper mute button, in that it also stops any media, such as music from an SD card, running. This then starts up, from where it left off, when you turn up the volume. This also leaves the nav announcement volume unchanged.
Q2: Pretty sure you can, but, without having a fiddle, not sure exactly how. Guessing it's in "options" when you start entering a new destination.