Aoa Brother,
1) G16B and G15A are both economical. I've a G15A I think I am getting something around 14-15 km/l on highway without atm, these are not accurate readings, however I'll be able to get accurate readings this week InshAllah will share with you guys. According to @sha2eb who had G16B he was getting 14-15 km/l in city. Though you should get 14-15 km/l with AC on a G16B or G15A.
2) For G16B I don't think firewall needs to be cut, though not absolutely sure about it. I can tell you that for G15A only the gardri was shifted a little to the left for my car, and that was the only alteration made.
3) As for default suspension, I am not quite sure, the engine is heavy and car drops from the front. You'll need to lower the ride from the back, since these engines are really powerful for such a small car you'll need to drop it compared to the default height in my opinion. I've not done any suspension work yet and can still touch 180 km/h but the car feels unstable from the back.
4) Double Hauda! Kuch samjh nai aai might wanna elaborate.
5) If you plan to go for CNG then the mileage you are asking might be difficult to attain. However theoretically you should get good mileage on CNG if the swap/engine is good. Should get 150 km+ easily.
6) Lower cost wala scene mushkil hai though depends what you term as lower cost. I got the engine for 70k, keep a figure of 80k for the engine in mind. My total cost of the swap without any suspension work was 1.20 lac.
7) My daily drive is 72 kms uss pay tau no panga, yet to take it on a long drive.
8) Don't go for a cone filter, go for the original air box filter that comes with the engine and use stock muffler of baleno or cultus, and you'll probably have a soundless car.
9) Without stage 1 upgrades, reborn and onwards beat kerna shaid zara mushkil ho, though I don't race so can't say for sure if the swapped khyber would be faster. However 1.3 coralla and 1.6 altis were not able to catch me, considering the fact that my ride height from rear is higher than the stock height cos no CNG cylinder (point being that weight distribution across is not right). And I touch 100 km/h in the 2nd gear. So it will be fast exactly how fast that I am not sure of.
As far as the denting and painting goes, adil can guide you much better than me.
Regards,