Is it possible to completely mute the microphone using the HIT button, i.e when using Talk mode or while tuning my guitar? Or it possible to pre-select the volume of the microphone whilst in Talk mode?
No, unfortunately not.
When you press and HOLD the HIT/TALK button and enter TALK mode, all effect output is muted and the mic input is passed directly to the output meaning that it is just a dry voice. The vocal EFFECTS are muted, but it is not possible to mute the vocal path entirely in this way.