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I agree with everything you've said. Answers are all yes, except #2, you don't need to do any more diagnosis. Wow, it would be easy if everyone did as thorough a diagnosis job as you!

The aftermarket cats don't last nearly as long as the OEM ones because they don't contain nearly as much of the rare-earth ingredients (you get what you pay for and all). 5 years might be typical for a life. The unhealthy MAF certainly could have contributed. That is why it is bad to go on driving for a long time with the CEL on: you may know you can ignore the original reason it came on but if something else goes bad you have no way of knowing and that can do the harm.

The rear O2 sensor should always see a very lean condition if the cat is working because the cat should use all the rest of the Oxygen burning up the remaining hydrocarbons. The fact that the cycling to higher voltages is present in the rear O2 sensor signal means both that the sensor is good and the cat is not doing its job.
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