If the opnsense interface on the WAN VLAN has a public routable IP address there shouldn’t be a problem with double NAT. Double NAT should only be a problem if they have a crappy ISP that’s using CGNAT.
Edit: never mind, I reread your comment. We’re saying the same thing essentially.
Not so sure about that. It’s one of the hardest failures so far. It’ll probably make lists including Atari’s ET and the Ouya for quite a while.