Whether IPv4 or IPv6 does not negate the need for a firewall at the ingress/egress.
I mean, clearly OP doesn't need to worry about the lack of NAT, but not having gone through all of the things listed on that list, I would assume that they all require an active participant behind the NAT/firewall, and would still be required for just a plain old stateful firewall (assuming you don't have the ports open) when you want to communicate with some other endpoint behind another firewall.
Not necessarily. I mean if someone's setup really is "just NAT" then you can just send packets with the addresses of their internal hosts to their router and it will route them.