I answer my question after my experience. Thank You usmanuetian for giving the idea.
1: What, if any, documents do I need?.
None.
2a: Cost of Petrol from Lahore to Islamabad?.
My car, Cultus 2005 carby, gave me 11 to 12 KM per litre. I was driving on 5th gear and maintaining 3000RPM. Speed should be approx 110 at this point. Window open too and of course, no A/C.
Take that in consideration and divide that by roughly 400km and you get your answer. Approximately 3700Rs. of petrol. from Lahore to Islamabad.
2b: Cost of CNG from Lahore to Islamabad?.
You are deeply unlucky if you travel between Monday to Wednesday as there is no CNG available in Lahore so you will have to spend petrol till Bhera to get CNG. That is the only point that you start getting CNG. As for price, not more than 2000Rs. of CNG. A/C off and driving at 110/120km/h.
3: Do they ask for license?.
No.
4: I have heard that CNG is available on motorway everyday of the week. True?.
CNG is available everyday of the week in some stations.
Now, what the hell is tanker gas?. One station, while I was returning back to Lahore, said that it will give full millage but wont show full points. What kind of BS was that though I did got 130Rs. of CNG from there as I had the CNG filled to full in Islamabad/Bara Kahu.
Some stations, having same rate of Petrol prices, they fill 4.86 litre of petrol for 500Rs. and some fill 4.89. WTH is this?.
How much CNG can the cylinder in cultus take?. I for the first time in my life saw the price sky rocket to 750+Rs. in one of the stations in Islamabad. Certain PSO. 
