The aluminum crossover tube is aftermarket?. There is an air flow sensor that is installed in the intake pipe.I would check this sensor first, take it out clean it with a contact cleaner in a can(aresol).put it back in the intake and check all connections. the reason for only one side of the exhaust being a little dirty is because it is a single exhaust into dual tips, a little more exhaust comes out of the one side is all. With the one coil pack gone and replaced, the code was probably picking up a miss fire due to not enough spark in one of the cylinders, this causes unburnt fuel to enter into the exhaust and into the cat. you replaced the coil pack and the code went away, now the cat has to burn out the rest of the contaminates , its going to take a little bit depending how long it was running with this code present. The black film is probably reminance burning out of the catalytic converter.
Being that it is very cold were you are and there is a higher concentrate of ethanol, i would try using slick 50, or even higher grade fuel stabilizer with a full tank of gas . Mid grade unless the car requires a higher octane. When the temp warms up use methyl hydrate (gas line antifreeze) to remove any moisture in the tank. Check the manual and make sure the car doesn't require premium fuel. If so this might be the problem. My 94 De-ville required premium and when I run a lower grade gas, I lost some power, and pick up. Only use recommended octane for the vehicle as it retards the knock sensor if you use lower grade fuel, hence the fuel stabilizer.
I hope this helps, but as it sounds, I wouldn't worry to much.