Real problem?? prolonged use of CNG or Poor quality of petro

I recently encountered a problem with my CULTUS 2007 CNG.Here goes the story:

It was all fine and the car was running absolutely fine on CNG uptil due to unavailability of CNG I had to run the car on petrol for a day. It took some time before getting started on petrol when I converted it from CNG to petrol on that day.
The day passed fine when the car was run on petrol the whole day.

The real problem started on the next morning.

The car couldnt start on petrol.. when it is recommended in such a cold weather to start the car in the morning on petrol..

The next step was to call a CNG mechanic......

He came and tried but to no avail.....at that time he said the problem was accumulation of carbon in engine and valves were not moving cause car was run for a long period of time on dry fuel that is CNG ..and I accept that...
Anywayz the car was taken to his garage where he succeeded in getting the engine run and a lot of smoke was coming from the exhaust...he charged RS.400 and also changed the plugs...but also said take it to a long drive on petrol so tht engine wud get 'cleaned'.....but I didnt.....

In the evening on the same day the car refused to start again ! crying

Next my father took it to another mechanic and he was able to start the car after using some tuning spray.....

Next morning the car again refused to start......

This time mechanic#2 was contacted and the second solution was sorted out as opening the half of the engine and changing the head and removing the carbon waste..... The cost for this was RS 5000

By this time the reason for this problem has been mentioned as prolonged use of CNG but when the engine was opened it also came out that poor quality of petrol ruined it....as some petrol was also taken as sample and it was of different colour than normal petrol but there is also a possibility that it was old petrol staying in the petrol tank for a long period of time .....

So my question is what in the real problem....can poor quality petrol cause too muc damage to your car.......or the actual problem was running the car on dry CNG for a long time....

Please comment..........