edit, delete, etc.

Can you do that with a letter once it is send? And the instance admin of the mirroring server can delete posts if that is legally required for some reason.

And how would that even work technically? Bulk import posts and spam other instances with mass updates? That would immediately detected as a spam-wave and blocked. And back dating technically new messages is also not exactly a great thing to allow.

Other implementations of nomadic identity like Hubzilla get around this by letting you run two accounts in parallel and syncing them from your main account, but they will also not back-port old messages before you linked up the secondary account.

Basically anyone with some experience with federated systems agrees that importing old messages in bulk on account migration will never happen, and I don't really see an issue with that, since messages are not lost.