Would it be possible to reinstall an instance without backup or only old backup?

Let's consider this horror scenario:

Suddenly Lemmy.ml fails, and the admins realise with a shock, that the backup didn't work properly.

What would happen if Lemmy.ml was setup from scratch or only old backup where there were only a small fraction of the users present?

• Communities/posts/comments that don't exist anymore after the reinstall → Obviously they can't be accessed anymore, and I guess this will be federated to other instances (they won't also be available there anymore)
• Communities/posts/comments of Lemmy.ml users that were made on other instances → These are stored there, they should still be accessable, but
  • Would the unavailable users just be linked to nothing
  • Would the users names that were available in the old backup be linked back to Lemmy.ml? Would these posts again be visible in the Lemmy.ml user history?