Several questions

[simoncn]

I don't think Album:Composition would be useful. You could try Composition:-Album (that's colon minus). The minus removes the Album tag if there is a Composition to replace it. The results could be a bit surprising but might be close to what you want.

As I have said, Group and Composition are not the same thing. You can make them the same quite easily and this will work most of the time. To make them the same, set Group.default={Composition} in tagValue.