I think it's pretty obvious that Konohamaru learned Rasengan easily BECAUSE of Naruto's kage bunshin trick. Just as Bud said, Naruto was the first one to discover that trick, which is why it was considered a much more difficult jutsu to perform before him.
Also, Kakashi doesn't have a problem combining elemental composition with chakra form. The reason he can't make a Rai-sengan is because the two are simply incompatible. A swirling ball of chakra doesn't evolve with just ANY element. Naruto's wind affinity is the ONLY reason he was able to develop the rasengan further than anyone before him.