Two commentaries abt Чтo:
1) I see it among conjunctions (union / link words), even in cases where it is clearly a relative pronoun. For example: Я не понимаю то, что ты говоришь. For me it is clearly a pronoun: I don't understand that, what you say. Different case is: Я знаю, что ты прийдешь, obviously a conjunction.
2) Even used in cases where clearly it is a conjunction I see it suppresed sometimes. For example: Я знаю, (что) ты прийдешь
As for the first question it seems being in "no man's land" in this dichotomy and it is perhaps only matter of words; for the second question surely will be some rules. Won't they?
Tks