That's actually one of the thoughts I had was to do something like that. Helps that I have a tendency to make unique jingle variants all over the place myself due to sometimes wildly different samples being utilized, kind of like how the later SNES Donkey Kong Country games do it.

The catch is how to handle the same song in multiple groups...

If I have everything start from a zero memory location and fill in the blanks, it requires a good deal of 65816 code to pull off and needs to be updated for every pointer-based VCMD that gets accounted for (there's a running tally of which ones to do), but it will do the trick. I can also do something similar for SFX groups that get swapped in and out along with their samples.

Without this, it's duplicate data galore due to pointer differences. Also, I may need extra RAM to track which music and SFX IDs are actually loaded to avoid complications if I were to automatically track this stuff.

And the original Super Mario World can be my test case, since I have exact IDs. And it has a huge default music group set to really test it out, although it has no global songs by default.

Varies on a case-by-case basis.

Seems like quite the ambitious project but the amount of work you've done so far makes me confident that'll you'll get it fully completed.
If its about making tools easier and having more features for both the casual and more advanced hacker then I am all for it.

this, what we all lived for

Immensely appreciate your endeavor at this! Some features you're implementing are absolutely must-have and are sure gonna help many porters around.