Responding without reading everything so I apologize if I'm duplicating. Yes a U.S. citizen would have the same treatment according to all my information. I do understand the problem with declaring cash when coming in to Argentina. It probably isn't that they want to break the law, but they don't want the word out that they are traveling with a lot of money--they want to be able to get from the airport to their home..
If it were me I would come in to Uruguay and cross over. Of course then you'd still have to get it into Argentina but I think your chances would be better, or arrange a place to store it there. In any event I think that would be easier. I will add that it is an outrage that this is happening. Confiscation of property that is legitimately owned is an outrage. My cousin's husband, a police officer, used to carry marijuana. If he wanted to arrest someone all he had to do was "find" marijuana in the guy's car. And if they keep the car, the police department sells it and keeps the money. So there is plenty of motivation for corruption. They tried that on someone I know about in Brevard County, Florida, and he wasn't putting up with it, told them he'd see them in court, they better start getting their evidence together (!) and they just gave him the ticket he needed to go pick up his car.
