Players ruining the fun for others seems to be a part of most online games, it's just something that has to be worked around.
As for a PC leader screwing others out of advancing in their job by setting no quests, that shouldn't be an option. If the leader doesn't speak with the quest NPC's then the quests given would be default/random. (Again more quests would be needed) My thought was that a ruthless leader would assign more difficult tasks for the 'recruits' to complete, making it more difficult for them to move up the ranks... So, there'd be no "No jobs for today" option available.
I think a term limit would be a good idea for the PC Leader. After the term, the 'Leader item' would be deleted from the players inventory and a place holder item given to the PC to indicate that they were just recently leader. The leader position would then be given back to the NPC, and another player could challenge the NPC for the lead. If that player succeeds and becomes leader, on acquire of the 'leader item', a script is fired to delete the holder item by tag from the module thus removing it from the previous leaders inventory so (s)he can challenge for leadership again in the future.
As for guards, perhaps they need to be toughened up a little.