At this time, my understanding is that content is nominally mirrored -- not "mirrored" in the sense of the word any IT pro would use. Should an instance go down...
First, the two would coincide. There's nothing at this time "destructive" in starting up another m/ethfinance on any kbin instance, or lemmy instance, or anywhere in the fediverse. It would live there just like r/gaming and r/games and r/truegaming do, and users would gravitate toward the community best fitting them.
Threads/comments/external links/posts (in the microblog) are cataloged by any other instance that properly federated and subscribed to the magazine from the point at which a user subscribed.
So, let's say I or someone else signed up over on new kbin server readit.buzz, we'd have to register on readit as a user there, then browse to and subscribe to ethfinance@kbin.social to start bringing over data. I've seen signs that the number of cross-subscribers (inventing this term) on a given instance helps prioritize the speed of syncing.
Caveats are as you've mentioned before: the data earlier than subscription is not brought over, original media is lost unless re-hosted elsewhere, and perhaps most importantly... the links are erased from time (eg. Google searches) so that archival threads, well, aren't. It's similar to the days of lost phpBB communities and forums in that regard.
Lastly, it is a very easy process to backup and transfer the database. However, because there is at least one step involved there with an admin, we cannot yet call it anywhere near "trustless"
I believe admin Sign-in With Ethereum (via Safe) partially solves this and a VPS company accepting Safe SiWE nearly entirely solves it. But that's a wish and a dream at this point. (Sidenote: what's the old dApp that used to parcel out CPU cycles for tokens, was it Gnosis? Or something else with a G?)