Good day,
In the meantime that this long series of sub-posts becomes a white paper, there is a new document available on CCO written by Lukas Krattiger that covers the Layer 2 and Layer 3 interconnection of multiple VXLAN fabrics. I feel this document is complementary to this series of Post 36, describing from a different angle (using a human language approach) how to achieve VXLAN EVPN Multi-Fabric design in conjunction with OTV.
Optimizing Layer 2 DCI with OTV between Multiple VXLAN EVPN Fabrics (Multifabric) White Paper
Good reading, yves