First of all, shall the feature set of the self-billing document be kept in synch with that of the invoice and credit note?

In addition, the question was raised today by France in CEN TC 434 WG3
How to handle Selfbilling in UBL for EN16931?
Do we use the Invoice / Credit Note documents, or do we have to add 2 specific documents (syntax binding)? 
see https://github.com/svanteschubert/CEN-TC434-WG3/issues/26
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It was pointed out that in Peppol, we use the same Invoice and CreditNote document types and add the direction of "Sender" and "Receiver" in the business message envelope (SBDH) instead. Peppol Specs: https://docs.peppol.eu/poacc/self-billing/3.0/