It looks like that P2A00 code can be caused by any of the items mentioned by the dealership, as well as other things on the intake side.
Since you mention a smog test, that means you're in California, right? Do you know if the car has lived most of its life there? I thought that most cars there just don't seem to rust that badly.
I live in an area that gets plenty of road salt during the winter months. When I had my '08 the catalytic converters did show plenty of rust, though it was primarily the heat shields. I didn't know the exact condition of the cats within the heat shields, but I never had any issues with the exhaust system that triggered a check engine light. While it may be hard to get the car on a lift to take a look for yourself, see if you can at least snap some pictures of the areas around the cats to see their condition. That part of the car isn't too low.
If this is urgent and you really want to put your mind at ease, I would say find a good local independent mechanic. Either ask people you know in the area, or spend a few minutes online to look for one with lots of good ratings. Whether or not you're an AAA member, you can also check their website for a list of their approved shops.
Here's what I think about the dealer and chain shop. The Infiniti dealer is accustomed to having people pay more, and they want to ensure that any problem areas related to the repair are addressed and the car is in tip top shape. The Meineke shop services more common cars and wouldn't want to drive away business with excessively high repair estimates. You could have Meineke do their fix first to see if it takes care of the problem, but you know there's always the risk that it doesn't get fixed right the first time. Proper diagnosis is the key, that's why I would say to get a third opinion.