Yes

Is your mechanic a "fire the parts cannon" kind of mechanic? It sure sounds like it. For example, before replacing the throttle body, I (and I'm not a mechanic) would have cleaned the throttle body and MAF sensor, first, then tested it. (It isn't clear that your mechanic went for "the simple and cheap" steps, first).  Any bidirectional scan tool (which a professional mechanic would have in their possession) could have commanded the throttle to open and close, by commanding the ECM. That would have determined if the throttle body was defective. 

Likewise, the knock sensor issue was, apparently, a symptom rather than a cause. IF that was the case, treating the symptom doesn't get to the underlying cause.

So, MountainManJoe is right to affirm your assessment. A restricted cat does create the symptoms that you describe. But what to do? Start with some catalytic cleaner in the tank, and take it for a good long drive on the highway. There's not much money involved in doing that, and it might help, if there's a relatively minor restriction in the catalytic converter. If the converter is rattling, well, that means that the honeycomb screen that is the central element of the cat is shattered, and nothing will help that.

Parethetically, get it diagnosed by a good independent mechanic. There are other causes for a P0420 code, such as an exhaust system leak. But, as MMJoe affirmed to you, you've identified the most probable cause for the failure.