Now do I still render the 5.1 track by itself, then render it again to downmix it to stereo, then render the video to NTSC DVD without sound, then combine them all up in DVD-A2 with a selection box for Stereo or Surround, or is there an easier way?
You can just use a 5.1 audio track and your DVD player can downmix it to stereo for you (no special handling- it just does it) OR you can render a separate stereo mix.
Extensive mixing articles can be found at Dolby.com. Worth reading - good 5.1 mixing is very complicated.