You may feel that Kamui is not worth voiding but I would void it, due to the fact that I see people are abusing that move (both versions). However, that would be a separate discussion.
Raikiri is characteristic of Kakashi, that doesn't mean only he can use it. It's just not something that is ever going to have a definitive answer.
Perhaps, but there are several variations of Raikiri at this point, it makes me wonder how it's even original. >.>
At least there's proof of other people using those variations.
Side-note: I believe most people just stick to using Chidori anyway. >.>
Which is why I said Raikiri and not Chidori, there is a difference as Kakashi is the only canon user of Raikiri.
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Raikiri and Chidori both are custom techniques of Kakashi (he created them based off of rasengan I believe), so of course the only ones in the canon series going to be using them are either going to be Kakashi or any of the students that he taught (I.E, Sasuke).
Raikari and Chidori are not a characteristic technique of Kakashi in the same sense as Mayfly being that of Zetsu. They both require the special manipulation of lightning release chakra in a certain way in order to use.
Mayfly is characteristic to Zetsu in the sense that it is almost like a special ability of sorts, much like how only Nara in the canon series use shadow imitation and what-not.
* Much like Kaguya's bone manipulation being slightly different than the standard Kaguya bone manipulation, Zetsu's ability to merge with the ground is likely a natural energy attribute (recall what Zetsu are made of by the time the series concludes. They are not merely Hashirama Mokuton clones) that is unique to the creature known as Zetsu.