P0421 and P0300 Codes on 2003 Mazda Protege - O2 Sensor or Catalytic Converter Issue?

Note: This 2003 Mazda Protege has only 50,000 miles and was purchased from a woman who likely rarely drove it above 45 mph. I initially experienced misfiring in one or more cylinders, confirmed by a P0300 diagnostic code. I replaced the spark plugs (which were fouled) and ignition coils. After clearing the codes, the vehicle ran smoothly at first—but I noticed a low idle when stopped at traffic lights. The Check Engine Light reappeared, and I received a P0421 code: 'Warm Up Catalyst Efficiency Bank 1 Below Threshold'. Most online resources suggest replacing the oxygen (O2) sensor before considering catalytic converter (CC) replacement. My questions are: 1. Did a misfiring cylinder cause damage to the O2 sensor? It seems logical that the catalytic converter would be fouled due to misfires. 2. Has anyone successfully cleaned their catalytic converter? I've seen YouTube videos showing someone adding a gallon of lacquer thinner to the gas tank for cleaning—though this method appears risky. 3. I have been using 85 octane Shell gasoline consistently over the past six months. Would switching to 87 octane improve performance? Is there any known issue with Shell gasoline in older Mazda engines? Thanks for your insights.