Well for starters it's a Sharingan NOT a Rinne-Sharingan, that fanmade translation is inaccurate and second that ability is not a combinding KKG. What they are doing is using their ability/talent to use nature as their ninjutsu. Just like when Hebi Sasuke shot his Fire style in the air to create real natural lightning, that was a lightning that had nothing to do with his chakra base lightning style but was simply a byproduct of his Fire style. Lastly, nobody copied any bloodline limit. All he did was use logic. It's the same as you making ice by putting water inside of a freezer.Zetsu got that ability from Kaguya. Kaguya's son/will is Zetsu. Kaguya has the Rinne-Sharingan. No one has combined KKG. In this series. Ever. Not once. Sharingan can't copy it. Sharingan can copy every jutsu. Suddenly in his adult years Sasuke learned how to combine both. Not practical/possible if it's never even hinted to be possible.
Fixed it for you. And to add to that it's really not entirely doujutsu based.- Certainly it's The Sharingan's Interference.
Then Zetsu happened.- Only a Sharingan User has shown a feat akin to this.
@Bold is complete fanfic. Senjutsu chakra has nothing to do with Kirin. Manga prove this and what you're saying is a misinterpretated fanfic:- Hebi Sasuke used The Curse Mark proven to be Senjutsu to create that lightning(Fire-Dragons enhanced by Senjutsu). It's Senjutsu Enhanced Raiton, KKG is element + element, miles miles difficult to perform without a Ocular Aid proven in Manga.
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My point is that both Zetsu, Kaguya, and Sasuke in their own way used organic elements without being able to use that kekkai genkai.
Zetsu moved the roots without being a bloodline limit user in the DB, Kaguya moved organic ice without being a Ice bloodline user, and Sasuke created Ice without being an Ice bloodline user as well.
Sasuke used wind style to freeze his water style jutsu thus creating Ice. Just like how Hebi Sasuke used his Fire style to heat the temperature in the sky creating Lightning. This is a simplification of what I've been stating during the whole time.