as you mentioned that from factory it came without AC being vx variant.
can u please share which AC did you install and did you engage any dealership for AC installation or got it done from any other place.
Alto has electronic TB and RPM is controlled by ECU compensating for any additional load. yes when AC is turned on the RPM do slightly incrase and can reach upto 950 to 1000.
Regarding the fuel economy, if you use it for very short trips, grocery runs and household chores, driving in 1st , 2nd and 3rd gears then max you can get it 16/17 km to a liter.
however if you take it longer journeys, keeping it in top gear and no hard pushing it can return you up 26/27 with AC on all the time.