Re: Mapping plan
The other two posters here covered alot of the broad strokes with the routing issues. 6 to 4 translation/mapping can happen at an endpoint router like the one at your home or a single site office at low/impact cost, when you are routing to one of your primary links. That is because the v6 address block is being routed to your internet connection from the outside. Since the IPv4 address space isn't assigned in the same structural hierarchy it appears the UN/ITU proposal wouldn't be able to to efficiently map to one block, it would require blocks in each division of the hierarchy for the legacy address space. That's where your big chunk goes.
The IPv6 space is STUPID BIG. Losing 25% wouldn't be a non-starter in and of itself, but the rest of the proposal appears to have other huge problems beyond the address space useage.
Unsurprisingly, the idea of reverse mapping the address space is not new. I set up one up in two days when I started breaking in my first IPv6 hand-off. The trick is that efficiently mapping address spaces in a trivial way leads to inefficient routing. The working group I was in was playing around with creating virtual IPv4 tunnels that overlay the IPv6 network. The key thing is you'd want your IPv6 block hanging as close to the tier 1 space as you can, as you will be bouncing traffic off of it in a somewhat inelegant fashion. It does work without breaking much, at least when the entire internet isn't doing it at once. You can spin up an Amazon tiny if you want to try it for yourself. It wasn't as horrible as it sounds as the IPv6 part of the routers were already dual stack anyway, and IPv4 stuff doesn't freak out about address translation that often these days. The project never went anywhere because it didn't get buy in from the people who really matter, the people who slinging networking silicon. If Broadcom, Qualcom, or Cisco don't back it, it's not happening.
That won't fix the IPv6 roll out though. IPv6 was and remains a lesson in failure. Too much forcing academic theory, not enough requirements analysis, and some bad assumptions on the wisdom of encouraging 25 year support windows for your core internet routing hardware.
I hope that IPv7 or IPv8 routers are all software running on upgradable hardware with the expectation that anything that's running older than last years LTS with the latest patches will be voted off the island. We have two perfectly useful broken Internets for anyone that doesn't want to keep up.