If we were to stay in Kingussie for a few nights, where is the closest place to rental a car?
Probably Inverness, but possibly in Aviemore, but while National shows up on the map, when you check their website, Aviemore is not listed. Avis is at the Inverness Airport. Sixt is at the Inverness airport or city centre. Hertz is Inverness. You could try Enterprise. They may offer the same service that they do in the US which is to meet you with the car.
Ask your B&B or hotel if they know of any place. Sometimes there are local enterprise that you might not know about. There is an email on this page that you might try.