But isn't it possible to have a router convert from an IPv6 address
with provider bits to an IPv4 address by simply stripping all but the
lowest 32 bits from the IPv6 address?
The hosts on the LAN remain entirely IPv4, but the addresses that
leave the router have the provider bits added by the router.
You can't strip a non-zero part of IPv6 address, as this would cause
transport layer pseudo-header checksum to fail.