The registry doesn't contain cmdline options now, or at least they aren't used, that was a launcher-only thing. And depending on what build you use (3.6.9 for instance) the sound and video registry settings should be defaulted if they don't exist. If there is no video entry in the registry then the game defaults to OpenGL 640x480 at 16-bit color. The sound will be OpenAL by default (even when both DS and OpenAL are supported in the same build later on) and will pick the safest sound device by default. The input (joysticks) is the only thing that isn't quite handled automatically, though the new input code I've been working on should take care of that. So, the launcher would/should only be needed if you wanted to change the default settings. Otherwise the game shouldn't need it.
In regards to "b", correct. The only thing to note would be that the launcher can and will make modifications to that file, so you can't really consider any options in there as something that won't ever change. It's a good place to get in your initial defaults at least however, and an easy initial troubleshooting point for you if end users have issues after using the launcher.