Technically, it's not a split. These are two different words, i.e. negative particle ни and pronoun кто in the required case.

All the examples are correct.

First о is short (like English shwa), second o is stressed (long).
Would it be correct to move the preposition?
Я у никого не был. (I haven’t visited anybody.)
Он с никем не говорит. (He’s not speaking to anybody.)
Я не обсуждал это с никем. (I haven’t discussed this with anybody.)
Я бы никогда не сказал такую вещь о ником. (I would never say such a thing about anybody.)