I’ve done a lot of SysAdmin and DCOps stuff in the past so, thought I’d give you some plausible suggestions (haven’t dug deep into Lemmy DB stuff and DNS/Federation of the stack, so not sure all is practical).

Scenario 1 - Preserve and merge when access is restored

Setup

• Spin up two VMs/VPS (or one that has enough grunt for two Lemmy servers). Call them robak.slrpnk.net and slrpnk.net and point DNS appropriately.
• Pull federated content from other instances and place it on robak, set as read-only.
• Sync important comms to (new) slrpnk.net without content.
• Allow users to sign up, vetting as possible (all mods). Keep a list of those that are vetted (call it vetted.list). Inform all users that any non-vetted users will have their content dropped when access is restored.

Merge!

• Once access is restored, ensure that (old) slrpnk.net is set to read-only.
• Schedule a maintenance window (announce more time than you are likely to need).
• During the maintenance window, put (new) slrpnk.net into R/O, or just block external access.
• Query the db on (old) slrpnk.net for all users.
• Subtract the vetted users from vetted.list from the list.
• Drop all records from the resulting list of non-vetted users from (new) slrpnk.net.
• Insert the records from vetted and new users (those without conflicts) into the DB on (old) slrpnk.net.
• Validate that everything is working
• Cut over DNS and spin down the new VMs/VPS.

Scenario 2 - Server is in DC or Admin able to facilitate access

• Get a db dump/backup.
• Spin up temporary slrpnk.net on a VM/VPS.
• Use backup of temporary server to restore data to original, when possible.