1) Does that diagnosis sound correct?
Seafoam sounds like a good way to go, but in this case it isn't going to cure the ill. The throttle body itself needs to be cleaned. I found this video which should show you pretty much how to clean it (as long as your engine is the same as the G35 in the video). Since the engine has reusable gaskets, as long as you are careful with the removal and installation of the throttle body (TB) you won't have any issues. You are just trying not to damage it, which in actuality shouldn't happen with the TB removal. If the TB to manifold gasket is dried out and cracked, it will need replacement, though.
The reason I said Seafoam isn't going to do much for you as the mechanic suggested, it's because adding it to the gas tank will not clean the TB. It will clean the fuel system and injectors, but won't make it to the TB because the top end of the intake tract is dry (meaning gas is not found there). You could possibly use Seafoam using this method, which may help some, but even in this case, it wouldn't get to the TB, so you'll need to use the carb cleaner method to accomplish.
2) Is it possible that the mechanic unintentionally caused this? (Maybe they got some dust on the throttle body or something?)
You never know about this as who knows what goes through the mind of a mechanic, but for the most part, mechanics are not going to do something to your vehicle, especially if this is a mechanic which you've known for a while. The reason I say this is because, while they want business, repeat business is where the money is at. If they start screwing their patrons over by doing things to their customers, they will soon find that through word of mouth they will run out of customers and business. I'm not saying there aren't shady mechs out there, I'm saying that they are usually few and far between. I would personally stay away from shops which have the moniker of "The Honest Mechanic" or some such, but that's just me ;-)
3) Is it true that Nissan and Infiniti throttle bodies should not be cleaned by someone who is not an expert (i.e. dealer) due to it being a delicate operation and easy to mess up and require some sort of electrical reprogramming?
I personally have not done a TB cleaning on Nissan/Infiniti vehicles, but looking at the video I posted above, it doesn't look any more awkward than any other TB I've seen. As long as a person is careful (as in not banging the TB around), there should not be any problem with a layman cleaning the TB. You do want to be careful with not over tightening the four Allen head screws which attach the TB to the intake. Other than that, ensure you are using a good brand of carb cleaner, such as CRC and you should be golden. The only things I'd do differently is to wear some nitrile gloves to keep the cleaner off of your hands and don't use a rag to clean out inside of the TB. Instead, utilize enough of the cleaner to flush the residue off of the TB. Just be ready with a couple of cans of the stuff and there shouldn't be an issue. The carb cleaner will completely evaporate, so it won't leave any residue behind on the engine and such, but put a rag under the TB while you are at it if you like to catch the cleaner and gunk as you clean.