1) If you do anything to the audio (add a second track, crossfade, mess with volume) it will render down to 2.0. This is where I recommend using Vegas (by way of Hypercube Transcoder) to re-create the audio back in 5.1 using the 2.0 mixdown as a guide, then muted out of the final output stage. This will also take care of those pops you here when you make a clean cut sometimes.
2) If you doubleclick on the bar above the preview window it will fill it to the full screen.