Answer to problem number one is that the millage will come down in the summers not just because of the A/C use but also because of the high temprature even wid the same preasure U get less gas in the Cylinder. So dont worry. In the winter the cars which deliver 140 Km come down to around 100 to 105 Km.
As far as the power output us concerned yes Wid CNG U loose a lot of power so this is somthing U need to live wid.
I dont know Y did ur mechanic not fit ur car wid advancer.
It is very important that you get it done as timing has a serious effect on the performance, and now the mechnics are trained so well in wiring, I dont think it shud be an issue.
Take ur car to FTFS Faisal town main road, and get an opinion from thr as they have vast experience of dealing with efi cars.