When we migrated here I set up like...5-6 other SquaredCircle communities/magazines on lemmy/kbin-mbin instances for the purposes of testing federation back and forth across numerous servers to try and find something that wasn't gonna gimp half our discussions (which is why we ended here since Lemmy.zip only defederates from a small handful of instances [like those who post CSAM and the like] and the response time was much better whereas a lot of other instances were taking multiple days to pull the data in).
Most if not all of them and the accounts related to them were set to delete once we settled on .zip (assuming said instance admins did indeed delete them lmao) so newcomers wouldn't find a dozen SC's and get confused.
That being said at no point was there any sort of 'mirror' or anything of the like (at least outside of the normal federation process if you wanna count that technically as mirroring the data). We officially kept the kbin.social community as our backup in case shit happened here and lemmy.zip ended up having worse downtime but once kbin.social died (and this place proved to be more than stable enough to not worry about needing a back up) we removed the sidebar link to it.
As far as NOSTR goes (I have zero knowledge on it so I don't know how it works) would the magazine need to be hosted on the mbin community or does NOSTR pick up federated ones through mbin? Like how instead of accessing lemmy.zip/c/memes you can use lemmy.zip/c/[email protected] or whatever to get that instances stuff, does it let you put in whatevermbinwebsite.com/c/[email protected] or anything?