I have read everything, and it does seem tricky. I know that you know this but it might be good to get back to basics:
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A ‘Work’ represents the creation of a new piece of music (even if it’s a small change*).
e.g. In the situations you are describing, could another artist ‘cover’ the new work? Or would they just be ‘covering’ the original? -
A ‘Cover’ is a performance of a work. If another artist is following the arrangement and has swapped out instruments (assuming no music has been significantly re-arranged for these instruments) and/or is changing ‘how’ they are playing it stylistically, it is a cover.
I don’t add many works, so I’m not sure, but some cases you are describing may have works added for convenience and convention with some styles and artists. Potentially to make use of specific work relationships. I don’t have a problem with this. If you need to add a work to store something then go for it.
On the flip side, if the new work is duplicating all the original work relationships, e.g. lyricist, writer, arranger, with just different recordings attached, it seems pointless to me.
*doesn’t really apply to popular music, where I would rarely add a new work instead of a cover