but switching to ski for Norway would be exceedingly weird.
Realistically it's not possible, not that it might be weird. You can't switch country just like this, at least when it comes to racing. I mean if your old ski federation agrees, and gives permission, it can be done just like that, but for anyone but old racers, who have no perspective and chance for results, it's highly unlikely old federation will just say "sure no problem go on, here's our permission". Without permission it's few years break (not sure now if it's 2, 3 or 4) required, and only then you can start racing for new federation. That means two things... one, you are NOT racing anything for that time period. Not World cup, not European cup, not anything even remotely connected to FIS. Even FIS citizen races are out of question. And second, you start with no points, which means you first need to get enough points to even been able to start WC, and when finally on WC, you start with number 70.
So unless US ski federation would give her permission, it means it would realistically never be an option to race for different country.