Are there portions of this path where you have to ride on the road ?
My wife and I biked along the Mosel River from Trier to Koblenz and then south along the Rhine River. While there may been a very short stretch or two of sharing the road with motor vehicles along the portion you identified, I don't recall anything of significance. My wife MUCH prefers riding on dedicated bike/pedestrian paths and this trip met her standard in full. It's a delightful part of Germany and a great way to see it.