I dont see excess oxygen getting into the exhaust system. The exhaust would have to be pulling a vacuum.the cat actually melts down..A bad O2 sensor can do that. O2 sensor can be the root cause of plugging a cat converter. All the excess fuel buring in the converter, cokes it up. Check the plugs to see if one bank was running rich.
An 02 sensor can cause this but exhaust leaks (between engine and cat) can do this as well. It allows excess oxygen into the exhaust and the cat works "too" well. Bad wires are also a big problem (excess fuel in cat)
plugged cats tend to affect all cylinders anyway and not 1 or 2 as in a miss
If you suspect the converter. Beat the hell out off it. Preferable after removal dump the crust & crap you loosen and put it back on. That's a quick/temp fix if its a plugged converter.