> I've never seen a mobile phone AP offer IPv6 to clients, but if they do they have to use SLAAC-compatible IPv6 NAT in that situation.

iPhone does that, and I’m pretty sure I’ve seen Android doing the same. The phone keeps a single /128 from the /64 assigned by the mobile network on its mobile interface and the re-assigns the /64 on the WiFi interface. No NAT is involved.