IMHO you do not need "active" IPv6. Most LANs (unless you have some switch-level filtering that blocks router advertisements from "unauthorized" nodes) can transport such IPv6 packets. Then it just takes being connected to the LAN and being able to send an arbitrary ICMP6 packet (which probably means being root on the attacker machine, not a very high barrier I'd say).

That's pretty standard where I am. Every Telstra router comes with IPv6 enabled.

No, you can choose if you want IPv4 or IPv6 or both, at installation time also if you want it in "autoconf-mode"