The train is far from full. I must have just missed selecting a seat on the ICE leg when I made the reservation. I later made another reservation for my father on the same train and was able to reserve a seat for him. And yes, all tickets are 1st class, Super-Sparpreis.
If I make a seat reservation and later try to claim a refund, where/how do I do this? I suppose at a ticket counter at the departing or arriving Hbf? The ICE leg departs Koeln Hbf and I could make it so I have some extra time to talk to an agent there and try to get a refund. Seems like it will be more trouble than it's worth, however.