Its also possible that Reddit rows back from the API change (at least temporarily). I hope they do, so that we have a bit more time to get Lemmy ready.
This is true, but there are two exceptions: admins have a purge action which permanently deletes content. And like others already said, when you delete your account everything that you posted is permanently deleted.
There were some suggestions to change the order of instances on join-lemmy.org so that smaller instances are shown near the top. So that could help to spread out users better.
If a post from a remote user gets removed in a community on lemmy.ml, that removal should also federate back to the user's home instance. At least in theory.
So if you host an instance with no communities, the only moderation you need to do is prevent spam bots or trolls from using it.