Regarding dignitaries, I've found them to be VERY useful late game keeping corruption minimized. Using their dignitary skill (the one that increases authority) allows you to eventually get to the demagogy skill, that reduces corruption by as much as 15 percent. I was making around 10000 a turn with sparta before I recruited some dignitaries and powerleveled them, after which my income returned to a respectable level (because 10000 a turn goes quickly when you've got 100+ settlements to manage)