1) Acquiring a language is easy for a native speaker. It happens naturally, without direct instruction.

I agree with the intent if not the phrasing. A "native speaker" presumably already knows the language. How about "Children effortlessly learn any language to which they are exposed. It happens naturally, without direct instruction."

2) Achieving high levels of literacy is hard, but for the most part it happens without direct instruction.

3) Acquiring the language of Standard English is hard, but for the most part it happens without direct instruction.

I'm not sure what you mean. Children of parents who speak standard English learn it effortlessly. It is hard for students who have not been exposed to standard English outside the classroom to learn it inside the classroom--but that takes direct instruction, so it must not be what you mean.

4) Learning to read complex texts is hard, but for the most part it happens without direct instruction.

5) Learning to write effectively is hard, but for the most part it happens without direct instruction.

6) Learning about language is hard. It does not happen without direct instruction.

Learning about language involves conscious knowledge and requires effort and direct instruction (or, in some cases, original research). Language knowledge per se (i.e., internalized knowledge that enables us to speak and understand language) is unconscious and effortlessly acquired. The conscious mind has no direct access to unconscious knowledge so acquiring conscious knowledge requires effort.

Perhaps you might replace "is easy" and "is hard" with "requires no effort" and "requires effort."

Dick