> I could still have 8.8.8.8 routed to me in v-next, and then later do something with 8.8.8.8.1
"v-next" 8.8.8.8 isn't the same thing as IPv4 8.8.8.8, so you now have two "interfaces" running different versions of IP with identical addresses.
> because most devices will implement the minimum. Android doesn't support dhcp6. My router is like most and has no option to enable dhcp6 or nat6 anyway.
Sucks to be you but irrelevant to the protocol.
> Ipv6 spec also says that a router could either default allow in or not via its firewall.
Just like in IPv4 - in the pre-NAT world (or if your ISP assigned you a block of v4 addresses), if you were connecting a device to the public internet it would have be assigned a routable v4 address and it would be up to the administrator what traffic they accepted.