You're contradicting yourself. Both he and you stated it was made up of at least two different chakra natures.
That was only a speculation, because of the suffix.
Since it is elemental manipulation and Katon cannot be manipulated, chidori, which sasuke developed a lot seems the logical choice.
Nope, that doesn't make any sense. Sasukes Katon: Gouryūka no Jutsu is also Keitaihenka and Seishitsuhenka. Sasuke gave the Katon-Chakra a new form with Keitaihenka. And you can't manipulate a Seishitsuhenka with a different one.
Wrong. It is how chidori was created and how Naruto makes his new rasengan techniques because it is shape manipulation he combines with his wind element.
That's why I said Keitaihenka alone. You should regard the whole context. The basic Rasengan and Odama Rasengan are just Keitaihenka-according to the Databook.
Again: C said, Enton= Keitaihenka.....
[ Keitaihenka /shaping variation
If someone radiates his Chakra in such a way that it accepts a certain form and even maintains it, then, one calls this process "Keitaihenka".]
....and Amaterasu. Means: Sasuke gives Amaterasu(Katon) just a new shape. Enton is like the FRS a combination of Keitaihenka and Seishitsuhenka.
Your example with Chidori isn't legitimate in this case anyway, because its Raiton. You can't manipulate a Ninjutsu-like I have said it before- with Seishitsuhenka. With Seishitsuhenka the basic characteristics of chakra change, but not its form.
Chidori: simple Seishitsuhenka of Raiton/Raiton-Chakra
Chidori Eisō: Raiton-Chakra=>characteristics of the Seishitsu Raiton+ Keitaihenka=> form of a spear.
Only Keitaihenka can change the form of Ninjutsu.