Personally I wouldn’t want to play on an anarchy server that runs 24/7, it makes more sense to find a few hours each week where everyone can be online at once so there aren’t so many disadvantages.
Have you considered giving police NPCs a buff when the owner of the town is offline? Make them stronger, faster, more perceptive, etc. With proper patrol lines set up and with enough police, any griefing should be noticed within fifteen minutes. At which point the town alert level is raised and the police/military reacts accordingly.
The grief detection system could be implemented very simply with a two-dimensional map laid over the town. Every point on the map is associated with a region of the town, and every region has a list of players with permissions. A server configuration option determines whether players can modify cubes without permission. If this option is enabled, a player who griefs a region causes the server to mark that part of the map as a crime scene. The police patrols then use the map to detect griefing.
The same system could be expanded into a robust event-system and could be used to implement tracking skills (for hunting).
Another option is to limit the amount of pillaging of storage containers so that only a fraction can be removed by players with hostile town reputation.
Yet another option is to give players a global reputation - if a player attacks towns, they gain some renown as a bandit. Eventually towns that have never before been visited will attack the player on sight.
With regard to construction versus destruction, you won’t have to worry about misplaced building blocks if you implement an architect system whereby the player designs the building before actually beginning construction. This is necessary so that automatons (like NPCs) can carry out actual construction, which removes the possibility of misplacement. In the early game the player can misplace things if they don’t design beforehand or if they are not careful. You could also let the player “follow” their completed design blueprints in the world, which is implemented by showing the player a semi-transparent hologram of their design in-world so they can follow it precisely. It can also give a warning if the player attempts to place a block that doesn’t match the design.
The architect system could be something as simple as a standalone voxel editor - like creative mode - with an export function. You could access it in-game with engineering paper and a pencil, or with a computer and CAD software.
I agree that building large structures out of sturdy material should be difficult, time consuming, and require the use of construction machinery, or lots of NPC builders.
