Collisions are just as likely with the 48 bits as they are with the last 24 bits, as some OUI vendors reuse mac addresses.

Using a mac address in the IP is terrible anyway from a privacy point of view, use a random IP in the subnet, send out a check to see if it's already used, if it is choose a new one. That check is already a feature of ipv6.

It would rather be an ICMPv6 neighbor discovery packet or something like that, IPv6 doesn't use ARP.

> Collisions are very unlikely, and a quick ARP packet should detect them if they do happen.

You can do that, sure. But it's more complex and slower.