If IPv6 is behind firewall, apps can't use it for P2P connections, so the major point of IPv6 network becomes moot.

And IPv4 NAT is actually possible to penetrate sometimes. So for some networks, IPv4 provides better P2P connectivity, than IPv6.

Not really, look at a great post on Tailscale blog how such P2P connection can be established: https://tailscale.com/blog/how-nat-traversal-works

With IPv6 you might have to use a hack to make firewall allow incoming packets (like sending a dummy UDP packet to the peer first). The firewall only read, allow or deny these packets. While NAT must mess with the message IP//TCP//UDP headers to work.