The only question you have that I have thus far figured out is that happiness is additive and province-wide. Click on a city and mouse-over the happiness rating; a tooltip will appear that gives you the breakdown. You have a series of positive and negative modifiers. If your total is anything other than zero, that sum will be added to your overall province happiness rating every turn. So, a positive rating will climb higher and a negative rating will continue dropping. Most of the modifiers I've found have been from buildings, garrisoning armies in cities, and province edicts (have to own the entire province to issue these). In my first Rome campaign, the happiness in Italia dropped like a rock (big province wide hit the turn after conquering another city in it) but climbed back up. One region got hit by an earthquake. In addition to damaging every building in the city (which cost money and time to repair) it imposed a -10 happiness modifier for two turns; I had to garrison both my armies in other cities of Italia for those two turns as a counter.