After reading your entire post, I have come up with the following:
Your point # 4 indicates that there is something bad on the wiring part. On petrol, the cng switch should indicate only petrol light. In current scenario, petrol is being mixed with cng during operation causing the engine to stall. Once you have corrected this problem, and there is still the same problem, get the ECM diagnosed for any problem.
ECM & EFi system diagnosis:
When the ignition is set to 'ON' position, the check engine light should blink just once and stay 'ON' until the engine is started. If it is so then the ECM is working fine. You just have to follow the procedure to retrieve the codes, which is mentioned in the top post above, and then refer to the table for code specific information on the same post. After successful completion of this step, you should come up with clear picture of the entire EFi setup. If you find out at this stage that the ECM is faulty, you have to replace it. For price/availability related queries, refer to page 3 for contacts list (link mentioned in the top post)
Fuel pump & Injector:
The pump should prime (briefly run) 2 seconds when ignition is set to 'ON' position (you can hear humming sound coming from the pump). It is a very preliminary check whether the ECM can control the fuel pump relay. If this is not happening, either the wiring has some issues, or the fuel pump relay is not working. If the pump is working, then get it's pressure checked. Also get the injector checked for any problem.
If the check engine light is always 'ON', your car must be fuel guzzler on petrol (black smoke from exhaust due to rich mixture), as any of the system fault is prohibiting the car to work optimally and your car is running in fail-safe mode.