[sciurius]

As itsme wrote, making copies has several advantages. You can change the key per setlist, but also other annotations like how the song should be performed etc. Changes to the ChordPro file contents still end up in all copies.
But I guess your 'problem' is a bit more subtile, given the phrase "I wanted to get away from the 2-4 different db entries per song."
Yes, making copies will imply that you'll get more than one db entry. And you must not forget to make a copy first before starting to adapt the metadata.
So, what I think would be the most elegant and user-friendly (but probably hard to implement!) approach is a sort of copy-on-write: When you try to change metadata or annotations, MSPro will ask "This song is used in different setlists, do you want to keep your changes local to this setlist?".
As a consequence, when removing a song from a setlist, all local changes will be lost.
And this not only applies to setlists, but also to collections, albums, and so on.