Naruto Chapter 626

[Y]

Naruto doesn't have to "sustain" the Kagebunshin. They exist independently of him. I'm not even sure if you killed him that the clone would dissipate automatically. That being said, Neji comments that he is searching for Naruto's "real body" while attacking the Kagebunshins during their fight, so clearly he believes that there is a Prime Naruto to be found and killed. Also, even if your theory were true, Naruto clearly does not behave as though it is (he has the clones work to defend Prime Naruto, and exploits the fact that he does so by disguising one of the clones as Prime Naruto during the Neji fight). This is an implication for a much different manga than a kids' battlin' coming to explore. Might I recommend the "Madrox" mini that Marvel published a few years back for this concept explored in a more mature context.

Also the Sharingan cannot differentiate between Kagebunshins and the true body, except insofar as Sasuke's insight could predict which body the clones were going to defend.

EDIT:

As was pointed out in IRC, you do in fact have to "sustain" the Kagebunshin; Kakashi comments that because he is weaker than Naruto he cannot keep the Kagebunshin up for long.