NOx is a byproduct of imperfect combustion due to nitrogen present in the fuel and air, 'high temp' just means that the reaction does not happen at room temperature. An excessively lean running engine produces more than normal but all real world combustion forms NOx, which is scrubbed by the converter.
Injector open electrically could also mean it's not fully functional mechanically. Could be leaking, stuck partially closed or only partially opening. It's common for them to get clogged or wear out. If the injector is not working correctly for whatever reason the ECU tries to compensate, which would throw off the timing of properly functioning injectors or ignition. Does the engine ping?
EGR seems like a good thought. I'd check the O2 sensor and probably (on principle for me) replace the plugs, wires, cap, rotor. Check for air leaks in the intake after the AFM (or whatever air metering the Subaru uses). I'd guess the catalytic converter is old and replacing it will likely get it to pass, but that might not the root problem. Still, could end up being how you fix it when the alternative is a $500 AFM.
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