"3. How do you create multiple audio tracks for a DVD (again, like the EditDroid set's isolated score)? Can the audio tracks be for select scenes only? i.e. - Is there seamless branching for audio, as well, that would not require two complete audio tracks but one base track with minor branches between the two? "

DVD-Lab can do this. It won't be seamless, but say that Chapter 7 was a segment with a secondary soundtrack. You would create a second video segment with its own soundtrack, and tell DVDLab to play the second chapter seven when designated. One caveat: The audio filetype must be the same. If the first audio track for the movie is PCM, then the first audio track for the extra scenes must be PCM as well.

When I made my prototype SW DVD, the second soundtrack was the commentaries, which altogether comprise about 20-30 minutes of audio. I had chapter selections that would play the video with the second sountrack, and then return to the commentary menu. You can create a second soundtrack that is silent except for sound at the designated spots, and force an audio track switch for those segments. This would probably be a lot more "seamless".