Brother if the phenomenon is only with AC on then simply change your compressor good kablo one will cost u 3500 rupees in Karachi.
For third point check your ignition / spark plug wires and distributor, the car is not getting enough power / current.
To verify that you can drive on CNG and Petrol the shifting point on petrol would be better.
For mileage u should first of all clean the transmission filter and change the transmission oil.
After that check the routine things like air filter, fuel filter, valve clearance and get a compression engine check if possible.
Is your car powerful enough to go on high speeds like 130 km /hr & above .