Completely off topic, but why does “an European” sound wrong? Like I’ve always said “an American” but never “an United States citizen” and it just doesn’t sound right.

Because congress lets him with the IEEPA and the Tarrif Act.

Yes, he could. It would definitely violate a bunch of laws leading to impeachment by Congress (in theory).

https://natlawreview.com/article/can-president-impose-tariffs-without-congressional-approval