There's no need to post old posts out "as if they are new". They just need to be hosted by the new server, and returned as part of the right collections when someone requests those collections. They do not need to be sent out at all, because they are not new.
Posts are "linked" with accounts in the sence that the actor id contains a URI, and if served up from a new address, that actor id can end up pointing to a location that does no longer exist. But Mastodon also stores the signature of the posts. So a simple solution to this is to also include signed assertions about the transfer of an account, and serve up the signature with the posts, and optionally perhaps wrap the whole thing in a "local" "Announce" activity which would mitigate some compatibility issues with clients that might object to an object being included in a collection with actor id's from a different domain.

This is not a complicated problem to solve, and one entirely orthogonal to the protocol, which just sets out a very simple framework for the distribution of activities and defines a basic set of activities. There's nothing in the protocol which even requires protocol changes for this.

> and since Mastodon doesn’t support backfilling

Mastodon backfills all kinds of things. There are just limits on it that would need to be eased, and limits on what of that it will serve up unauthenticated. Then these objects would need to be imported. This is a very soft barrier to poke at since Mastodon already has code dedicated to processing posts from other instances, just not anything specific to migration.

> (and likely never will) it’s just not going to happen.

Mastodon also "likely never would" support quote posts not that long ago, and yet I see quote posts all the time, and can quote post myself as well.

There are multiple forks of Mastodon, and many other pieces of Fediverse software, and if/when the demand is significant enough people will sooner or later make this happen. To make it more concrete, my girlfriend is involved in setting up a service running various Fediverse software, and I can guarantee you from first hand knowledge that it will at some point add support for migrating posts to Mastodon whether or not the core devs want to. No protocol changes required, and in fact most of the work can be done outside of Mastodon entirely with only minor tweaks to Mastodon itself. Have you looked at the code? I have.

> This isn’t even getting in to the logistics of redirecting replies and likes/boosts.

You don't need to. You're unnecessarily overcomplicating things. You can obtain those when doing an import, and you can accept that either the original server will redirect them, or it won't. Currently they won't. It's not by any means a disaster if new boosts or replies for an old post using the original actor URI disappears into the ether - what matters by far the most to people is that they are still there in their timelines. Frankly, I'd be willing to bet - it certainly applies for my old posts from before I moved to my own instance - that to most people it's irrelevant whether the posts are accessible via ActivityPub at all, as long as they're still visible in their timeline on the web.