My understanding is that if an instance suddenly dies, all the federated instances that subscribe to its communities will still have the text content because they store copies locally. So knowledge should not just go away. Media is a different story though.

I think new posts/comments in those communities would then not federate at all anymore since the host instance would not acknowledge them. So the communities turn into isolated local ones.

If the host instance comes back and the communities are re-created, they’ll be empty on the host instance but I think other instances won’t delete the old content unless explicitly requested.

On iOS swiping from the sides works for back/forward.

On the macOS 14 preview, the app gets back/forward menu items with keyboard shortcuts.