I'll actually keep this short. See.. it all depends on how you word it Kid.
1.) Yes and no. Some you must do to advance the story. For example to get from chapter one to two you must A.) Get a second class B.) Randomly defend the wall from PvP 4on4 to advance.
2.) Yes.. some more than others.
3.) I would think that the reward merits the difficulty.
4.) 4-20 minutes for any quest.. so about 10 minutes. Depends on how you play. If you have to fight to get the prize.. it'll take longer. If you just move object A to B or fetch items/info it's way faster. Learn to use your map quickly.


