2005 Citroen C3 Engine misses

Engine Performance problem
2005 Citroen C3 4 cyl Two Wheel Drive Manual 90000k miles

Hello. The car (1.1L engine) was running fine up to this point although it was in severe need for tune-up & oil change. One time on attempting to start it, it started but it seems to miss one (could be two?) Cylinders. That morning we noticed a bit of smoke from the exhaust which I thought was blue-gray.
I replaced the spark plugs, air filter the coil assembly (4 coils and leads all in one assembly). The old spark plugs had a wide gap and some black on them (but no oil) with the exception of #1 which had a normal look apart from the wide gap. After replacing all this the problem is exactly the same.
1- Is it possible that if could be fuel? How can I check the injectors?
2- Could the timing belt or valves have anything to do with this?
3- Could the ECU be the problem? Is there a way for me to measure the input from the ECU to the coil assembly?
4- To do a compression test the car should be warm, but I am afraid that if one of the cylinder is not firing, raw fuel is going to ruin the catalytic converter. Am I correct?
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