After WWII, well, why wouldn't they get to keep the entire Nan'yo? It was populated by a majority of Japanese, after all. I know the Northern Marianas look nice on a map, being just south of the Ogasawara Islands, but it seems like it was either all or nothing for Nan'yo.
The biggest problem with the Northern Marianas is that it was too close to Guam, one of the most key US military bases. Even the friendly post-WWII Japan wouldn't be allowed that.