The carbon buildup is usually from oil from the CCV. The oil from the CCV is typically from crankcase pressure, assuming you have no other issues. Idling shouldnt cause crankcase pressure. If you are really worried about it you could consider a catch can.
2. I still dont understand the logic of city driving having any effect. TBH sport or comfort probably makes little differences, the car is made to be driven in the city. But sport is killing your fuel economy im sure and possibly making it worse
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