Yes, unfortunately 18 hours is not enough time to get the answers you need, If you can, go to the BATF website. Serial number is not a real issue, you create one and etch it on, better if you take the can to a laser engraver and have it done, looks better.
I am not a lawyer or ATF Agent, but I believe POSSESSION is not illegal until you mount it on the firearm.
Before people jump on me: OWNING a short shotgun barrel (< 18 inches) is not illegal, but once you mount it on the receiver it is. Otherwise it's a paperweight or a pipe.
It's costly, but you could ship it in your baggage, then "surrender" it to a class III dealer, then complete the application and pay the fee. After it's approved you pay the Dealer his charge and it's yours.
Now the issue is NOT BATF, it's State Department because you are importing and I have no idea what you'd need to do to import one item (Other than stick it in your suitcase and keep your mouth shut). Realise you can NEVER sell it because it's not imported legally.
All in all, I thought the same thing, in one country, I could walk in with my rifle, and walk out threaded with the suppressor installed for $700, BUT I could not bring it back into the states legally. Also, since I have an FFL, I have "more" to lose, based on home and business.
Also, I saw a couple of rifles I'd love to own at a great price, but again, I cannot IMPORT them without a license.
Perhaps you could declare it on your customs forms as as souvenir or a paper weight?
One more item: Probably the best advice: Don't buy it yet, check with State and BATF, do what you need (you are not intentionally violating a law) and see of your PH can bring it with him or send it with someone coming over in January for the trade shows. I'm sure that's covered somewhere or else the guys overseas would have a trunk filled with the things to sell.
Think Night Vision Devices, BATF has no jurisdiction on those but State does because you're importing.
New England Guns is in Maine, I believe and they have export permits because a PH was looking at buying a double rifle here and taking back to Africa with him. The shop would handle the transaction legally but it was going to cost something like $1500 - $2000, If I remember correctly.