I'm largely with you... I would think they'd take the IPv4 block and have a direct/virtual block that just extends it to more addresses... so it could be an IPv4 NAT or IPv6 direct.

like 1.1.1.1/192.168.45.4 ... for a router that understands IPv6, that's the direct route to the sub-network, otherwise it will have to use IPv4, and the subnet route is treated as NAT and otherwise isolated.

To me, that would make more sense... then internal IPv6 might be practically limited to 10. and 102.168. in the nearer term, but adoption would be MUCH simpler in practice, and distribution would mostly already be established, however unfairly, but can then be broken into single addresses and vNext adoption could be that much quicker as a result of piggybacking.