Please understand that I'm being mostly tongue in cheek . I just thought it was a wildly weird and fish-out-of-water thing, to put so specific a topological aspect into the introduction of the story. It would have made sense to me if Sophia, Liam or one of the kids had said "Oh, I'm feeling a bit lightheaded!" and then Carl says, "Yeah, it's the altitude here at the airport, but don't worry, the city itself is closer to sea level." That wouldn't register as out of place. But as written it was jammed in there with a crowbar which makes it awfully suspicious. 3000m altitude is both really really high and really really specific.
As for the German highway, there's no way the topology and local vegetation ==> Germany, and, as to the no speed limit thing, I don't care how good the handling and suspension is on your Porsche/Audi/BMW supercar: You'r not dropping 3,000m in 25 minutes safely, so the conversation when they arrived at the house would have revolved around "Carl is a fucking maniac and there's no way we or the kids are ever getting into the car with him again."
(This of course assumes you're a mentally stable warm-blooded mammal and didn't wait for the chauffeur)