I’ve worked in the auto parts industry for 12 years now, and this sounds odd to me.
Did the dealer tell you 6 months for the seats to come from Japan? Or are you just assuming they are coming from Japan based on needing to wait 6 months for them?
Your car was built in Smyrna Tennessee. Seats are one of the worst parts for automakers to transport because they are bulky and very hard to ship efficiently, and because they have to be “sequenced” - delivered to the plant in the same order for colour, power options, etc as the cars are being built down the line, which changes based on orders until the last possible minute - usually when the body hits the paint shop. So seat suppliers usually put their plants right next door to the automaker’s plant to reduce the shipping time and to be able to respond to changes in orders. At the very minimum they need a sequencing plant nearby.
It is possible that Nissan service just doesn’t have seats in stock yet, as the Altima coupe is new and seats aren’t a part which typically needs to be replaced so early, but I’d be surprised if even that’s a 6 month wait.
Regards,
Mike