i dont think that Kakashi can use the rasengan...
thats why he invented the chidori, i think its some sort of spinof idea to the rasengan...
i think some 1 says it but im not sure

who the 3:rd person is i have no idea...

EDIT:
ok so i didnt read all of the post but that last idea sounds about right to me [img]i/expressions/face-icon-small-smile.gif[/img]
And pure utter speculation, maybe it was Kakashi's friend Obito, and thus being unable to copy it because he can't simply copy chakra control, he developed chidori to try to mimic it?