If you're going from Cesky Krumlov to Salzburg take a bus. Renting a car in the Czech Republic and dropping it in another country will be expensive. Rent the car in Salzburg if you must have one. This should also eliminate the issue of the vignette.
Get an IDP.
Call your insurance company and get coverage. Some offer it, some don't.
Forget Google, use Waze. It's much more current and smarter. It can show you routes if you don't have the vignette. It also shows construction and traffic cameras, which there are a lot of.
If you don't understand how to drive on the Autobahn, don't. In fact, you have to understand the speed limits everywhere, because most of them are not posted, but are enforced. In cities, once you pass the yellow sign, it's 50 km unless posted different. Back roads are "normally" 100 km. The organization I work for says it takes an average of 45 days for Americans to adapt to local driving in Europe.
Be advised that driving to Hallstatt means parking a couple miles out of town and walking in. (Personally I think this is one of the "sights", like Neuschwanstein, that's vastly over touristed. You'd be a lot better off going to St Gilgen, and taking the ferry to St Wolfgang or Strobl.)