Could it be possible to add the 'Disc number' / 'Part of set' tag to Mp3tag? In my opinion, this is the most convenient way to deal with multidisc sets (double albums, boxed sets and so on).
Also, I accidently deleted more than 10 CDs when hitting the Delete key by mistake. You really should enable the 'Show message when deleting files' option by default!
you can add user-defined tags via the View, Tags ... menu with the latest development.
I'll enable the delete message box by default to the next release. In the meanwhile, you can have a look at your Recycle Bin. Maybe you'll find your lost albums there.