I don't think a cat that is a couple months old is going to die.

If you used an o2 sensor from the dealer, there is almost no chance that it is bad by now, meaning the mixture should be controlled accurately so as to not poison the cat.

The advice you picked up about coolant temp should have you looking for the engine not warming up.

So it is likely it could be the rear o2 sensor.

3% fuel trim ... your intake is well sealed, maf reasonably accurate and injectors working well. If you had a leak in this part of the exhaust you would hear it.

With an OBD2 tool (I really like PCMSCAN) you can read the actual o2 sensor voltages in real time. Do not hook up a meter probe to the o2 sensor connections!