I've never seen any choas in the coach. There's a little light over the seat saying it is reserved. If someone else sits in the seat, they have to expect to give it up to the reservation holder.
I think (maybe Andreas can answer this for sure - perhaps call DB) that German Rail could sell a ticket or reservation for a purely domestic Danish trip at the counter, but have chosen not to do so online.
I think if you call German Rail, they can advise you, maybe even sell those reservations to you. Their number, in Germany, after your international access code, is 49 (country code for Germany), then 1805-996633. When I have called in the past, the person answering didn't speak English, but there were English speakers available. The first time I called I said, "Do you speak English", figuring if she did I wouldn't have to do it in Germany (I speak a little). She said "Moment", and put me on hold a few minutes for an English speaker. The second time I just started in German, figuring once she heard my thick American accent she would switch to English if she could. She didn't, and I got my answer in German.