So the issue is that the import command was meant for non Sponge created world's, like Bukkit worlds. Also renaming the world folder doesn't rename the actual world so it's conflicting with the main world. It has to be renamed in the level.dat file. Couple options here. First you could drop the world in the imports directory that's in the server root, remove dim1 and dim-1 seeing as you don't plan on using nether and end for that world, and restart the server. From their it should..(should being the key word, it's not full proof, make a backup first)..rename the world to something unique, move to the main world directory, and automatically load it. From there you can use the rename command to name it to something you prefer. Other option is to use something like nbt explorer to rename the levelname value in the level.dat file, rename to world folder to match the value, remove dim1 and dim-1 like mentioned before and copy it to the servers main world directory before starting the server.
It's a lot to take in I know but it's working around a few issues with the API not having native support for modifying servers dat files and the implementation not knowing how to handle world name conflicts. This should be less complicated in 6.x builds.