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Even weirder: when the router forwards the packet from the host to the Internet, it already sees both the source IPv6 and MAC address, so it could store them.

Maybe there are some weird situations where a host that just got its own IP address starts proxying for a third node that wants return packets to asymmetrically bypass the host?

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This is what I don't understand. I'll be the first to stand up and say there's a lot about IPv6 I don't know, but why can't/doesn't the router learn how to talk to the host when the host sends that outbound packet?

I'm guessing it's one of those completely over-engineered bits about IPv6 that is that way just because they wanted to engineer in so much complexity almost for the sake of it

Generally IP does not make that assumption. If I remember right (it's too early), having to go through neighbor discovery combined with a switch doing some special processing on ARP/ND packets protects against identity hijacking in the LAN.

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