I have sonica 2007 model. I dont have such issues .
I will answer your question one by one
1) your temperature sensor might be faulty.
2) must be issue with radiator. i had similar issue, my ac used to trip. upon self investigation i learned that my radiator was not cooling the car because of which the car used to trip the ac to ensure the car does not overheat. you might also want to check the water in the radiator. There can also be a problem with the radiator fan.
3) get it checked from ac wala.
4) mileage depends on how and where you drive. use light acceleration, minimum brakes. my also gives from 12 to 18, depending on how and where i drive
5) bilal gunj seems to be the option unless a local mechanic can repair it.
you can visit asif (03064412682), adjacent sarhad hotel , gurumangat road ,lahore. i have got my sonica's radiator fan replaced from him. plus i also got my sonica's generator/alternator repaired from him.