You can probably do a one way car rental from Las Vegas to Durango, and you can definitely do a one way car rental from anywhere to Denver airport, but the drop-off fees for a one way rental can be expensive. If you're going to start your trip in Durango, why not just fly to and from Denver? That way if you rented a car you could pick it up and return it in one place, and avoid the drop off fees.

Getting your bikes on the plane from Germany to the US and back is not going to be cheap. I think Lufthansa charges 150 euros each way. I would go to the websites for the individual airlines that you are thinking of using, and keep the charges for bicycles in mind when you compare airfares.

I think you mean that your bicycle tires are wide but not knobby (knotig, knopfig). I would not plan on finding one way bike rentals in Las Vegas or Denver, but you could definitely rent bikes in either town if you picked them up and returned them at the same place.