The scene depicting the team hunting for Vibranium in the ocean has one of them saying that the machine is the only one they got, heavily implying that it's their first time using it and it's a test run to gauge its effectiveness. It's possible that they would've considered reverse engineering it once the machine proved its potential in locating Vibranium outside of Wakanda, its flaws aside (the machine is shown as being rather power intensive and shuts itself off, so reproducing the machine might probably need a little more thought than just remaking it as is no matter how valuable it may be in detecting Vibranium).
As for Riri, I get your complaint, but they don't show a scene of Riri returning to the US (only that she's preparing to leave Wakanda). I would think that it would be pretty hard for the US to ignore that Riri was explicitly shown attacking FBI agents and got kidnapped by Wakanda (they'll assume it's Wakanda since they're unaware of Talocan), but addressing the whole ramifications of that at the end is probably a little bit too much. The movie is long enough as it is.