or maybe at least the posts wouldn’t be replicated, but there would only be one community, and when you visit it, posts for the community are downloaded from your primary instance, as well as other instances based on some configuration that makes sense.
good point with being able to switch to another if a mod goes haywire. but i think that there just shouldn’t be any mods – or rather, the community members should moderate themselves via some algorithm that uses votes, discussion, etc. to hide/remove posts. you could choose to view a hidden post that has been downvoted a lot if you want.
to clarify, “admin of an instance” is different from “moderator of a community”.