Ok, just a short post-road trip update:
The road trip went fine, no problems at all. It was great, there were so many things to see and 3 weeks was actually too short (as are all holidays). But I really enjoyed it!
Returning the car to Adventures on Wheels wasn't a problem either. They inspected the car for damages and fuel and stuff, but everything was fine (except that I had driven more miles than agreed upon beforehand, so I had to pay a little extra). So my experience with AoW has been positive and I can recommend it if you need to rent a car for a longer time.