a failing crank position sensor in this engine would immediately set the check engine light, and a failed one will shut it off. What you are describing is classic symptom of GDI misfire and drop of rpm - the idle motor tries to compensate but the combustion pressure is not enough to raise rpm back to normal.
It would be ideal to test the system on a live stream - check all data and compare to factory values. You might need to get the injectors cleaned - they are mounted from the back of the head directly into the combustion chamber. the high pressure common rail is above them and its pressure pipes come to the high pressure pump on the side of the head.
The problem is that they operate at 1800 psi. I dont think any test lab in Pakistan can clean those injectors, diesel ones are easier because diesel fuel is thicker. These have extremely fine tolerances.
Your best bet and cheapest also is to buy a can of liqui moly intake valve cleaner, mix it with some petrol and feed the engine from the PCV port, this solution will hopefully remove the carbon deposits on the piston tops and also injector faces to some extent.