Every Vitz owner has the problem of its low road clearance hence it hits the jumps. You have two options to increase the height.
1. Change your tyres from 175/70/14 to 185/70/14
2. Install a packing in between rear shocks.
Do one of them and you will not face the same problem again. If you do both of them then you will get the height of Corolloa xli.. ANd just forget if there is any big jump under your car even with 5 persons on board.
About the average, do throttle body service, clean plugs and change air filter. Use Toyota Genuine oil 5w30 that comes in a tin pack for better fuel milage.
Vitz is a very durable car but it depends on how you drive it. Best car for bad roads is Corolla xli or gli. Vitz does just fine as far as you are not driving it on 80kms/h on a broken road. Normal jumps are no problem for it. This car has 3 types of gears. D, S, B. D for normal driving, S for overtaking or climbing on heights, B for engine braking when you are returning from heights.
I live in lahore and on Canal road, there is a big hole in the road and i have hit it many times and nothing happened to the car. Once i was traveling with my friend on his honda city and when he hit the same hole, his right tyre's "Goli" was damaged and the whole alignment was disturbed. So it is a durable car but to be honest i dont enjoy the drive of my Vitz as compared to Gli when i have to go to my village with the bad roads. This car is not made to drive on Pakistani roads' 'khadday'.
About the pulley problem, not sure about it. Let other expert members answer that.