-I saw that train too, but it takes 14 hours (as opposed to 11) and is a K train rather than a T train (probably not a whole lot of difference, but T trains are almost always air conditioned IME)
-definitely just buy a one way ticket to DLC (as you noted, there is no savings in buying RT; plus schedule flexibility is always a good thing)
-and, if you're feeling really adventurous, you could always travel by boat (these prices seem high to me; probably an agent is involved).