Hello manx,
yes, supercard (as far as I know all versions, although I only have Supercard CF, which isn't available anymore; supercard rumble and the slot 1 versions don't work with gba files, but nds works) should work with ScummVM and all other nds-homebrew files. The new beta-version of ScummVM should work with all cards, for which DLDI-drivers are available, you just have to patch it with DLDI before (see:
http://chishm.drunkencoders.com/DLDI/index.html for infos how to use.)
I don't have experience with any other cards, but there is also a compatibility-list with recommended cards on agentq's scummvm-page at
http://scummvm.drunkencoders.com for the official, stable version, just in case, you don't want to use the beta-version.
About moonshell: it's a media player, you can watch videos (must be converted to dpg before), listen to mp3s, view images, etc, it's a pretty nice program)
hope I could help a little bit, just excuse my bad English, it's not my native language
