Quick answer: The O2 extenders won't hurt a thing (although, they can be a pain to modify AND to get fitted just right).
A catcon simply catalyzes unburnt fuel and byproducts of combustion (burns it) before it exits into the atmosphere. Nothing magical.
At 96k miles, I'm sure the dealer is full of crap. Cats last FAR longer than that (I've seen MANY with 300k+ miles), and when they fail, the code comes back instantly... nothing that will damage the engine.
P0420 and P0430 codes CAN have other causes (a stuck injector, a bad plug, dirty MAF, a vacuum leak, even a cracked manifold).
If it were my vehicle, in your situation, I'd do the following:
1) Nothing. Drive it. If it's running smooth and you're not seeing other symptoms, roll on.
2) Order the O2 extenders and fit them (know that they *may* not work, but in most cases they do).
3) Replace O2s (non-dealer parts).
96k miles - She's just getting broken in. Our 05 Frontier (same drivetrain) was bulletproof for 150k brutal, abusive miles.