I once used MP3Tag to fix some MP3s where the artist got screwed up because the track name contained a slash, and these files had already been put into separate directories by artist. Since you could match album even recursively in directories, it was easy enough to move all the files back to their proper place.
On linux, puddletag provides a decent GUI for modifying audio file metadata. For bulk editing / transcoding, parallel with ffmpeg and (fd)find works very well.
To avoid violating libmp3lame's copyright, you need to include their notice somewhere in your application.