Regarding this release:
Best I can tell it’s a actually a bootleg that’s ripping off the legit Gregorian artist’s cover style, and Magna Canta’s music. 12 of 17 recordings share names and acoustids with Magna Canta recordings (in fact, it’s basically Enchanted Spirits with some extra tracks).
What’s the recommended way to address this?
If you have ear checked that they are the same recordings, please merge the recordings and set the original artist as recording artist.
Add a release annotation in the ripoff release to explain.
I’m not telling which one is original and which one is ripoff.
But if these are re-recordings of the same songs, just link them both to the same works.
If the recordings are in fact the same, should I move this into the Enchanted Spirits release group, too, just marked as bootleg?
Do you know where the other 5 tracks come from?
Beware some of the 12 tracks don’t have same length, don’t merge those. 
Some of the others are mentioned here, I’m going to try to track them all down: http://slushaem.com/forum/viewtopic.php?t=294
The Magna Canta tracks are all <5 seconds difference, mostly 2 or 3. The biggest one is Hymn, which has that much variation already in the known tracks.