I think we should leave jerk-issues alone for now. The best solution to this problem is giving the leader the right to accept who joins his fortress and who doesnt, and also to ban people from it. That way, he can decide to only invite people he trusts or that the community trusts, and ban any annoyances. Ofcourse, that would mostly not apply for the first version, since there is only one fortress.
As for a more ingame answer to such problems, just have that making doors with private locks uses much more energy than normal doors, or requires special parts (mechanical parts, lock, key...) to be made, thus making locking doors a hard business. Sure, players won't be able to lock them when they just started a new fortress because they can't produce the parts yet, but when the fortress is still young, there are very few players, and they probably already greatly trust each other, since they decided to start the fortress together.
As for the mountain-digging, that would require so much energy it would take so much time to accomplish, the leader would surely notice what was happening and ban the player.
My idea of the jerk-problem in this game is, try to fix it indirectly. After all, this would be a very high pvp game. Dont give concrete limits to what can and cant be done, let the player decide between the benefits and costs of each decision. If the player feels like stealing from another player, let him. He'll just have to then escape before the other players decide to kill him, and can't return to this fortress, and probably won't be able to cary his own stuff away in his escape...