1. Yeah sure! It's just like any other class so feel free to add your own fields, or use the description field however you like (it's just an extra field which has no specific purpose in the code - you can set it to a string, or to a list or tuple of other information you want to access, for example). So you could use the description field as a genre field if you weren't confident about adding your own genre field!
2. All songs are stored in the playlist field of the ExtendedMusicRoom - you can fetch an individual song's MusicInfo via mr.music_dictionary["audio/music/nutcracker.ogg"] where the path is the one you used to set up that song in the music room and mr is the extended music room the song is in. Typically you have to loop over the whole track list though, which is what the get_tracklist method is for! It'll return all the MusicInfo objects used by the extended music room. Usually that looks like for song in mr.get_tracklist(all_tracks=True): and then you can access song.name and song.description etc.
Hope that helps!