* add another entry to the global routing table thus nullifying a benefit of IPv6
* cost a bloody fortune (approx £5,000 per year)
* is your mum able to ask for BGP peering
I am personally able to pay the price for PI but not everyone is and that is my fucking point. Why should your mum not be able to ask for a internet connection that is able to tolerate a failure?
There is NPT but it is simply NAT via another name.
I've been running IPv6 at home for quite a long time. I passed the WAF at least two years ago.
I haven't tried it myself, but I guess one option would be to use DHCPv6 with low lifetime timers? That way router could stop advertising the prefix from upstream that no longer works.