[KevinH]

For your 3., agreed but unrecognized dc:identifier schemes are not going to be supported in the MetaData Editor. Especially since for EPUB3, the scheme is no longer recommended and instead a urn: based prefix is added to the identifier.

For custom things like that, you will have to edit in the opf being careful to properly add any needed prefix definitions if needed.

Edit: but if there is a popular dc:identifier scheme that is not already in the list of recognized lists, I could add it to the list easily.

Edit2:
Looking at that image you posted, that dc:identifier is a uuid. It has nothing in particular to do with Calibre per se. Any software can generate a random uuid identifier. The scheme here should be UUID.