Or, a composer releases a version as the primary artist, but for some specific region the vocalist is credited. Currently considering this case, Tracks #1 and #2 of the first RG is the same audio-wise as Tracks #1 and #3 of the second:
You merge recordings if they are literally the same audio. Not because they sound the same.
If the same tape has two different names on it, then that is merged.
If in one area this recording is credited to the band, another area credited to the vocalist, then as long as it is the exact SAME recording then it can be merged.
If someone listens to a recording, and makes a cover, then it is not the same.
If someone makes a recording, then puts out the instrumental version without the vocals, then this is not the same recording and is not merged.
Credits tend to stay with the original recording artist. Or at least the most popular\accepted name
Oh then, if they are the same backing tracks (karaoke) and everything in these tracks from start to end do sound exactly the same, then they should be merged, indeed, and such comments as ナルセーナver. should not be kept (you can merge targetting the recording without version comment, as it’s also the oldest).
oh got it, I actually randomly picked them for demonstation here without having listened to those, since I know he re-used instrumentals on some of his other releases