Gantz balls are just convenient plot device at this point. Its functionality changes whenever the author needs to create or discard a situation.
21 grams thing is, as someone says, pretty bs, and Kishimoto and Kurono's relationship is pretty much non-existent. The old Kurono (current one doesn't give a damn) only cared about Kishi's body, and Kishi treated Kurono as if he didn't even exist.
The aliens also treated "souls/information" as indestructible. However, by creating clones, the aliens also showed that "souls/information" can be replicated. Essentially, it is something that can be created but not destroyed, which contradicts their earlier method of proving that humans are simply things (a faulty method in the first place, but anyway). They couldn't/didn't destroy the information when trying to prove their point.