"utilizing multiple IPv6 addresses per interface inside your LAN"
Have you ever actually tried that
Not on my own LAN. But I have been using my laptop on LANs which were configured like that.
with increasing numbers of per-interface addresses?
Who says the number has to increase? Each interface should only have one static and a handful of temporary addresses. The temporary addresses are removed at the same rate at which they are added.
increasing numbers of AAAA RRs?
You don't put temporary addresses in your AAAA records. If something need a AAAA record, you use a static address for that. So it is static address on the server side and temporary address on the client side.
NAT66, adjusting only the high order ("prefix") bytes at the external gateway is vastly more simple
Sure that is the simplest form of NAT, which you can possibly do. However it does not give you any of the anonymity or topology hiding, which is often given as motivations for using NAT in the first place.
Yes, you can avoid renumbering that way. But there are drawbacks as well. You will break any protocol, which is not strictly a client-server protocol. And you will break any algorithm, which looks at assigned addresses and behaves differently depending on whether you have a routable IP address or not. Is this breakage of protocols you may be using on a daily basis really worth it, in order to avoid a bit of extra work in the rare case, where you may have to renumber your network?
the only requirement for NAT-friendliness is that the host implementations do not tell higher layer protocols the truth about what's in the high order bytes.
How would you make anything but a strictly client-server protocol work with that requirement? Let's keep it simple and consider just the following scenario. Two clients (which could be both on the same LAN or on separate LANs far from each other) each resolve the IP address of a single server. Each client contact the server and indicate they want to communicate with each other. The server informs each client about the address of the other, such that a direct communication channel can be established between the clients.