IMO this isn't debatable.
Part 1:
Option 1 because it's the only convenient way. You need a PC to put the files on the SD card anyway, so doing it on the Wii is redundant and unnecessary. Unless you plan on making some sort of Homebrew Browser style installer, but distributing the music for customs isn't exactly legal so
Part 2:
Option 1 and 2 because:
2 is possible as long as you've installed the DLC with the app beforehand. A custom app is only needed if you add new DLC to the SD card. So it's the obvious "normal" method.
1 is needed regardless to install new DLC into the game.
You'll always need a custom launcher or patch IOS37 in place if you plan on using an SDHC card with RB2 (and we end up doing it), but that's obvious.
SFenton:
There are ways to do this without unlocking all DLC for people. And it's always going to be the SD method.