Q: Sulphur blocking the catalytic converter

My car has the dreaded injector coil issue and I’ve already replaced it two times! Currently, it is running better but the engine management light comes on randomly. The dealership told me this is caused by sulphur blocking the catalytic converter. They blamed me that this is caused by the way I’m driving. I was also told by a tech that my car is remembering how it used to be driven (ex-fleet car with mostly highway miles). Now that I own the car and it’s not being driven as much as before (I just drive around town and shopping) this is the cause for the sulphur blocking the catalytic converter. He said it’s not a warranty issue, but instead the way it’s being driven. His solution is to desulphurise the car and this can be done by my regularly driving it on the interstate at 70 mph for 30 minutes. This seems rather impractical to me, plus such a bother! This was also questionable because I was driving on the interstate the second time the warning light had come on and was going over 70 mph. When I bought the car I was aware there was an unlimited mileage warranty, so I don’t think it’s unreasonable for me to expect BMW to honor this! If a car can’t perform well on short trips, then there must be something wrong with it! This is a petrol engine - not some diesel engine. In addition, I was told that the blocked catalytic problem can be caused by the injector coil. To add to my problems, when I got my car back from the garage, there were fresh scratches on the front, side bumper!! I will call BMW about this also. Would you please give me your advice, diagnosis, and solutions for my car?