[itsme]

about 6)
You can organize what you call "tags" with the custom group as proposed by BRX. Just add the tags you want to the custom group and check the ones you want to apply to a song in the song editor. You can apply more than one to a song. Or reuse an existing group that you don't need otherwise, maybe "Genres" or "Source Types". You can delete all the default values and create your own.

The custom group is different to the 2 custom fields: in the custom group you create a list of values that you can check or uncheck to add them to a song. The custom fields are just edit fields.

I agree with you that some more custom groups and custom fields would be useful. Also the possibility to specify names for the custom fields has been already requested before.