Public Order doesn't have a direct effect on your country. It simply prevents nasty !!Fun!! like riots and uprisings. Public Order itself does the following:
1. Prevents hostile elements from growing in your country (note: these tend to be interior, not foreign).
2. Keeps your citizenry in check (it will start to clash with Unrest if both get too high).
3. Slightly (by slightly, I mean roughly a -1/+1 bonus at each of the spectrum; very small) increases the chance of interior actions succeeding.
4. Decreases the amount of hostile events that plague you (haven't had alot of these in the game yet, but trust me; you don't want to).
Public Order won't give massive benefits for being super high; but what it will do is prevent high unrest from ending up screwing you over.
Unrest is simply the hostility shown your government by the different people in your nation, and has little to no effect on stats. High unrest will negate all the positive things you get from Public Order, make it harder for you to change your country (this one is very important; reform is only possible if the people of your nation are willing to let it, or they are so heavily controlled that there's no one that can stop you), and increases the chance that the faction(s) upset with you will try to mess up your day.
RL nations did use tariffs as a source of income, but its not a "apply this to your trade route, magically gain income" sort of thing. The thing about tariffs is you are reducing the amount of optimal trade YOU get from a trade route, since you are decreasing the profit that merchants get from going to it. It's more of a "you can use these to get more income than your rival and increase the value of domestic industry" thing. Keep in mind that tariffs are a form of government control, and control tends to result in reduced economic potential.